Appellants Roberts and Robison argue that the prosecutor (1) improperly bolstered the credibility of its chief witness with his plea agreement to testify truthfully and (2) implied in closing argument that a state policeman corroborated that witness’s testimony.
We reverse for improper closing argument.
FACTS
Early one morning in January 1976, an unexploded bomb was discovered in a building used by the federal government.
Roberts and Robison were charged with the attempted bombing. The government contended that Roberts had recruited Robison and John Adamson to blow up the building.
The first trial resulted in a hung jury, but on retrial the jury returned guilty verdicts as to both Roberts and Robison.
Adamson was the government’s chief witness. In exchange for his testimony in this case and three others, hé received immunity from prosecution for 10 felonies and was allowed to plead guilty to a reduced charge of second degree murder. His plea agreement provided that he would lose these benefits “should he at any time testify untruthfully.”
The entire plea agreement was admitted in evidence and, in closing argument, the prosecutor made devastating use of it. He told the jury that the critical issue was whether- Adamson or Roberts was telling the truth, that Roberts could be expected to lie to save himself, but that Adamson would not lie for fear of violating his plea agreement and that a state police officer, Detective Sellers, was monitoring Adamson’s testimony.
On appeal, appellants attack the introduction of the plea agreement, the prosecutor’s use of it in closing argument, and his exploitation of Detective Sellers’ presence at trial.
We hold the prosecutor committed reversible error by using Sellers’ presence to argue for Adamson’s credibility.
CLOSING ARGUMENT
In his closing argument, the prosecutor’s first theme was that the plea agreement, coupled with Sellers’ supervision, made Adamson a credible witness. He declared:
Throughout the argument, ladies and gentlemen, there are two points which I would like you to consider. I would like *533you to think about these two points throughout the arguments ... [of all counsel]: Either John Harvey Adam-son when he testified two, two and a half days he was on the stand, either he lied when he gave his testimony or the defendant Neal Roberts lied when he gave his testimony. The testimony is open as to Roberts and Mr. Adamson. They are inconsistent. One of them has to be lying.
So you must ask yourselves who would have anything to gain by either lying or who would have anything to lose by lying. I suggest to you, ladies and gentlemen, that John Harvey Adamson had everything to lose by lying. I want you to read the plea agreement that is Government’s Exhibit No. 1. I want you to read it from the first page to the last page. I want you to read it until you understand it.
In that plea agreement he has agreed to testify in four matters. He agreed to testify truthfully in all four of these matters. If he was caught lying as to material facts in any one of these matters, then his plea agreement would be called off. The charges of first-degree murder would be reinstated, and he would stand a very good likelihood of going to the gas chamber. If he would lose the bargaining power that he has, he has to serve a minimum of 20 years and two months instead of going to the gas chamber. This is what he stands to lose if he comes into this court or any other court that he has agreed to testify to and lie.
Detective Sellers has been pointed out throughout the trial as sitting in the courtroom during the testimony, particularly of John Harvey Adamson. I would suggest to you that Detective Sellers is not here on vacation. He had a mission to serve and that mission was to sit and listen to the testimony of John Harvey Adamson.
[DEFENSE COUNSEL]: If the Court please, there is no evidence of this, and I don’t know if Mr. Sellers is here on vacation or not.
THE COURT: Yes, let’s stay with the record.
[PROSECUTOR]: I submit to you, ladies and gentlemen, that he was here to listen to that testimony and make sure that—
[DEFENSE COUNSEL]: Object on the same grounds. It’s the same. It’s not in evidence.
[PROSECUTOR]: If Adamson lied, ladies and gentlemen, the plea agreement is called off.
We must examine the prosecutor’s comments to determine (1) whether error was committed, (2) whether it was preserved for appeal and (3) whether the error was harmless.
PROSECUTORIAL MISCONDUCT
We need not belabor the well-established principle that the prosecutor has a special obligation to avoid “improper suggestions, insinuations, and especially assertions of personal knowledge.” Berger v. United States, 295 U.S. 78, 88, 55 S.Ct. 629, 633, 79 L.Ed. 1314 (1935).
It is improper for the prosecution to vouch for the credibility of a government witness. Vouching may occur in two ways: the prosecution may place the prestige of the government behind the witness or may indicate that information not presented to the jury supports the witness’s testimony. Lawn v. United States, 355 U.S. 339, 359-60 n. 15, 78 S.Ct. 311, 323 n. 15, 2 L.Ed.2d 321 (1958); United States v. Lamerson, 457 F.2d 371 (5th Cir. 1972).
The first type of vouching involves personal assurances of a witness’s veracity and is not at issue here.
The second type of vouching involves prosecutorial remarks that bolster a witness’s credibility by reference to matters outside the record. See United States v. Garza, 608 F.2d 659, 664 (5th Cir. 1979). It may occur more subtly than personal vouching, and is also more susceptible to abuse. This court has declared that such prosecutorial remarks may be fatal if:
the remarks, fairly construed, were based on the District Attorney’s personal knowledge apart from the evi*534dence in the case and that the jury might have so understood them.
Orebo v. United States, 293 F.2d 747, 749 (9th Cir. 1961).
The prosecutor in this case referred to evidence not in the record by declaring that Detective Sellers was monitoring Adamson’s testimony.1 The jury could naturally believe that Sellers had personal knowledge of relevant facts and was satisfied that these facts were accurately stated by Adamson. In effect, the prosecutor was telling the jury that another witness could have been called to support Adamson’s testimony. This was error. United States v. Morris, 568 F.2d 396 (5th Cir. 1978) (improper to imply witness not called supports the prosecution), Reichert v. United States, 359 F.2d 278 (D.C. Cir. 1966) (improper to refer to witness’s statements not introduced into evidence).
PRESERVING THE ERROR FOR APPEAL
The error was preserved for appeal. Defense counsel properly objected to the prosecutor’s reference to evidence outside the record. The objection was sustained but the prosecutor persisted with his argument.
It would have been helpful had defense counsel asked the trial court to give the jury a curative instruction, but such a request is not necessary when the error is brought to the court’s attention and curative action is clearly called for. In United States v. Berger, 295 U.S. 78, 55 S.Ct. 629, 79 L.Ed. 1314 (1934), Justice Sutherland declared:
That the United States prosecuting attorney overstepped the bounds of that propriety and fairness which should characterize the conduct of such an officer in the prosecution of a criminal offense is clearly shown by the record. The trial judge, it is true, sustained objections to some of the questions, insinuations and misstatements, and instructed the jury to disregard them. But the situation was one which called for stern rebuke and repressive measures and, perhaps, if these were not successful, for the granting of a mistrial. It is impossible to say that the evil influence upon the jury of these acts of misconduct was removed by such mild judicial action as was taken.
Berger v. United States, 295 U.S. 78, 84-85, 55 S.Ct. 629, 632-633, 79 L.Ed. 1314 (1934).
Trial court judges are not mere referees. Johnson v. United States, 333 U.S. 46, 68 S.Ct. 391, 92 L.Ed. 468 (1948), (dissenting opinion); United States v. Eldrid, 588 F.2d 746, 749 (9th Cir. 1978). They play an active role, keeping the trial running efficiently with a minimum of error. Their control over closing argument is broad. United States v. Sawyer, 443 F.2d 712 (D.C. Cir. 1971)
A trial judge should be alert to deviations from proper argument. Because such comments have the clear potential for affecting adversely the defendant’s right to a fair trial, the judge should take prompt corrective action as appropriate in each case. See Viereck v. United States, 318 U.S. 236, 247, 63 S.Ct. 561, 566, 87 L.Ed. 734 (1943).
Vouching for a government witness in closing argument has often been held to be plain error, reviewable even though no objection was raised. See, e. g., United States v. Ludwig, 508 F.2d 140 (10th Cir. 1974). See also United States v. Carleo, 576 F.2d 846 (10th Cir. 1978) (court raised objection sua sponte).
HARMLESS ERROR
The error here may not be dismissed as harmless. The prosecutor did not limit him*535self to comments on the evidence. Under the standard of prejudice on the record as a whole, we cannot say the prosecutor’s comments were harmless. The government did not have a strong case. Compare, United States v. Giese, 597 F.2d 1170, 1200 (9th Cir. 1979) (overwhelming evidence of guilt). Adamson was its chief witness and the prosecutor acknowledged that the government’s case depended on his credibility.
We are not persuaded that the jury would have convicted the defendants had it not been exposed to improper argument. In similar circumstances, the Supreme Court concluded “prejudice to the cause of the accused is so highly probable that we are not justified in assuming its nonexistence.” Berger v. United States, 295 U.S. at 89, 55 S.Ct. at 633.
INTRODUCTION OF A PLEA AGREEMENT CONTAINING A PROMISE OF TRUTHFULNESS
In the event of a retrial, the district court may benefit from some guidance concerning the remaining two issues: introduction in evidence of the entire plea agreement and prosecutorial use of the promise to testify truthfully.
Adamson’s plea agreement provided:
It is agreed by all parties that the defendant shall testify truthfully and completely at all times, whether under oath or not, to the crimes mentioned in this agreement. This shall include all interviews, depositions, hearings and trials. Should the defendant refuse to testify or should he at any time testify untruthfully or if any material fact in the defendant’s transcribed statements given to the State prior to this agreement be false, then this entire agreement is null and void and the original charges will be automatically reinstated.
The issue here is not whether the agreement is admissible but whether Adamson’s promise to testify truthfully should have been excised before the exhibit was presented to the jury.
Evidence that a witness is testifying pursuant to a plea agreement is usually admissible to show bias. United States v. Goodman, 605 F.2d 870, 880 (5th Cir. 1979); United States v. Antone, 603 F.2d 566 (5th Cir. 1979). It, like other circumstantial evidence, may be rebutted by evidence of explanation. The plea agreement may be portrayed fully and placed in context so the jury is not misled about its terms or importance. United States v. Rosson, 441 F.2d 242, 244 (5th Cir. 1971), cert. denied, 404 U.S. 843, 92 S.Ct. 140, 30 L.Ed.2d 78 (1971).
Evidence is not admissible, however, simply because it is contained in or is offered to explain a plea agreement. References to irrelevant or prejudicial matters, for example, are often excluded. See, e. g., United States v. Arroyo-Angulo, 580 F.2d 1137, 1145 (2nd Cir. 1978) (fact that witness’s family is in protective custody). Repetition of a prosecutor’s assurance that a co-conspirator’s guilty plea was wisely negotiated in light of the overwhelming evidence of guilt is similarly inadmissible. United States v. Cochran, 499 F.2d 380 (5th Cir. 1974).
TREATMENT OF ISSUE IN OTHER CIRCUITS
Three circuits have addressed the question whether a plea agreement to testify truthfully is admissible.
The First Circuit has held that admitting a promise of truthfulness was not plain error. United States v.Miceli, 446 F.2d 256 (1st Cir. 1971). The Miceli opinion implies that the promise should not have been admitted over a proper objection, but the scope of the holding is unclear.
The Seventh Circuit has held that questioning the witness concerning a plea bargain promise of truthfulness was not vouching for the witness’s credibility because it did not imply knowledge of facts outside the record. United States v. Creamer, 555 F.2d 612, 617-618 (7th Cir. 1977). The Creamer court seemed to say it is impossible for the promise to carry an improper suggestion.
The Second Circuit has held that a plea agreement containing a promise to testify truthfully is admissible on re-direct exami*536nation to rehabilitate a witness. United States v. Koss, 506 F.2d 1103, 1111, 1113 (2nd Cir. 1974). The Koss court declared that the evidence bore on credibility because the witness would lose his plea agreement immunity if he committed perjury, but cited no authority.
THE TEST FOR ADMISSIBILITY
The trial court has discretion to exclude evidence of bias. It must weigh the probative value of the evidence against its prejudicial impact and against the possibility the jury will use the evidence improperly. United States v. Diecidue, 603 F.2d 535 (5th Cir. 1979). This test has been equated with the relevancy test performed under Rule 403 of the Federal Rules of Evidence. Id. We conclude this test applies equally to evidence offered to rehabilitate a witness who has been impeached for bias.
A strong case can be made for excluding a plea agreement promise of truthfulness. The witness, who would otherwise seem untrustworthy, may appear to have been compelled by the prosecutor’s threats and promises to come forward and be truthful. The suggestion is that the prosecutor is forcing the truth from his witness and the unspoken message is that the prosecutor knows what the truth is and is assuring its revelation.
Conveying this message explicitly is improper vouching. Lawn v. United States, 355 U.S. 339, 359-60 n. 15, 78 S.Ct. 311, 323 n. 15, 2 L.Ed.2d 321 (1958), Gradsky v. United States, 373 F.2d 706, 710 (5th Cir. 1972) (improper to imply government checks for credibility before using witness). We conclude that conveying it by implication is equally improper.
A trial court should be alert to the problem of vouching before admitting a plea agreement containing a promise to testify truthfully. The court should consider the phrasing and content of the promise to ascertain its implications and decide whether an instruction to the jury would dispel any improper suggestions.
USING THE PROMISE IN ARGUMENT
In addressing the third issue we consider the use which a prosecutor may make of a promise to testify truthfully once a plea agreement has been admitted.
The prosecutor may not tell the jury that the government has confirmed a witness’s credibility before using him. Gradsky v. United States, supra. He should be no more able to indicate that the government has taken steps to compel the witness to be truthful. Both of these arguments involve improper vouching because they invite the jury to rely on the government’s assessment that the witness is testifying truthfully. .
As Judge Friendly declared, prosecutorial remarks implying that the government is motivating the witness to testify truthfully:
are prosecutorial overkill. They inevitably give jurors the impression that the prosecutor is carefully monitoring the testimony of the cooperating witness to make sure that the latter is not stretching the facts — something the prosecutor usually is quite unable to do; If proper objection had been made to the summation, the judge should have sustained it; if matters had gone too far to make a striking of the remarks an effective cure, the judge should have instructed that the promise in the cooperation agreement adds little to the truth-telling obligation imposed by the oath; that the prosecutor often has no way of knowing whether the witness is telling the truth or not; that the books are not filled with perjury indictments of Government witnesses who have gone beyond the facts; and that an acquittal would not mean that as a matter of course the Government would seek such an indictment or even fail to make its promised recommendation of leniency. If prosecutors know that such instructions will be given, they will hardly be tempted to the excesses committed here.
United States v. Arroyo-Angulo, 580 F.2d 1137, 1150 (2nd Cir. 1978) (concurring opin*537ion). We agree. The prosecution may not portray itself as a guarantor of truthfulness.
Several other circuits have held that the test for improper vouching is whether the jury could reasonably believe that the prosecutor was indicating a personal belief in the witness’s credibility. See e. g., United States v. Bess, 593 F.2d 749, 756-57 (6th Cir. 1979), United States v. Ellis, 547 F.2d 863, 869 (5th Cir. 1977). This court expressed the same idea in Orebo v. United States, 293 F.2d 747, 749 (9th Cir. 1961).
JURISDICTION
Judge Wyatt argues that we cannot reach the merits of this appeal because the appellants have not been validly sentenced. We disagree.
This case was transferred from the District of Arizona to the Southern District of California for trial. Arizona judges presided over the trial in Southern California but sentenced the defendants in an Arizona courtroom, using forms headed The District of Arizona.
The dissent would establish that (1) Rule 21, Federal Rules Criminal Procedure, requires transfer of the entire prosecution and (2) subject matter jurisdiction is transferred to the new court. Both propositions are essential to the conclusion that the sentence of the transferor court is void because it had relinquished subject matter jurisdiction entirely.
Subject matter jurisdiction, also called jurisdiction over the subject matter, refers to the types of cases a court is authorized to hear. It is not transferable.
Every federal district court has jurisdiction over the subject matter of this case:
The district courts of the United States shall have original jurisdiction . of all offenses against the laws of the United States.
18 U.S.C. § 3231.
Rule 21 provides for change of venue. A criminal defendant has a constitutional right to be tried in the jurisdiction where the crime was committed. This “constitutional venue” right, like other venue rights, can be waived. United States v. Powell, 498 F.2d 890, 891 (9th Cir. 1974).
Occasionally courts speak in terms of jurisdiction when they mean venue. See 15 Wright, Miller & Cooper, Federal Practice and Procedure, § 3801 (discussing civil venue). This imprecision unfortunately causes confusion, but it does not convert venue problems into problems involving subject matter jurisdiction.
Assuming Judge Wyatt is correct that the transferor court cannot transfer the trial and retain authority to impose sentence, the appellants should have been sentenced in the transferee court. Neither the appellants nor the prosecution objected, however, so they waived their venue rights.
This court must reach the merits of the case because we have no basis to hold that the sentence imposed was void.
REVERSED.
. We reject the prosecutor’s contention that his summation responded to comments made by defense counsel during Adamson’s cross-examination and that it involved only proper inferences from evidence of Detective Sellers’ presence introduced then. His comments were neither responsive nor appropriate. A prosecutor may naturally feel compelled to respond to the argument that criminals receiving favors from the government in exchange for their testimony are untrustworthy. We suggest that he might appropriately explain to the jury the necessity of using unsavory witnesses. United States v. Armedo-Sarmiento, 545 F.2d 785, 794 (2nd Cir. 1976), cert. denied, 430 U.S. 917, 97 S.Ct. 1330, 51 L.Ed.2d 595 (1977).