OPINION OF THE COURT
Murphy, P. J.Defendant was tried upon an indictment charging him with third degree criminal possession and sale of a controlled substance. Both counts of the indictment arose out of a single transaction in which defendant allegedly sold one glassine envelope of heroin to an undercover officer. At defendant’s trial, the undercover officer testified that after observing defendant pass a glassine envelope to a woman, he approached the defendant and said "uno”, whereupon defendant, in exchange for a marked 10 dollar bill, handed him a glassine envelope. Immediately after this sale, a description of the seller was transmitted by the undercover to the officer who, minutes later, arrested defendant. Contrary to the expectation naturally arising from the undercover officer’s testimony, however, the arresting officer testified that a search of defendant incident to his arrest disclosed only 10 dollars in unmarked currency; neither drugs nor buy money was recovered.
To the aforementioned testimony, the prosecutor added the testimony of a third police officer, who was presented to the jury as an expert in street-level narcotics transactions. This officer’s lengthy testimony, spread out over some 17 pages of the record, went into considerable detail about how street-level conspiracies to sell drugs were typically structured: a "steerer” directed customers to a "pitcher”, who would obtain the drugs to be sold on an as-needed basis from a "stash man” and pass them to the buyer. The proceeds of such transactions were delivered with some frequency to a "money man” and might thereafter be laundered at stores "fronting” for the conspiracy. All of this activity was safeguarded by "lookouts” and supervised by "managers” or "owners”, who would periodically replenish the street-level operation’s supply of product. This particular division of labor was employed, according to the expert, to minimize the risk of detection and successful prosecution by reducing the probability that the most exposed members of the conspiracy would, if apprehended, be found in possession of either drugs or buy money.
*20Defense counsel lodged timely objections to the admission of this expert testimony claiming, inter alia, that the testimony was unnecessary to explain the simple hand-to-hand transaction at issue and that it would encourage the jury to speculate baselessly that defendant was a member of a well-orchestrated conspiracy to traffic in narcotics. The trial court, however, determined that the expert testimony was admissible for the limited purpose of explaining why neither buy money nor additional narcotics had been found on the defendant at the time of his arrest and so instructed the jury.
We are of the view that the trial court erred in admitting the expert testimony and that the consequence of this error was to deny the defendant a fair trial.
It is "settled and unquestioned law that opinion evidence must be based on facts in the record or personally known to the witness [citations omitted]. He cannot reach his conclusion by assuming material facts not supported by evidence [citation omitted]” (Cassano v Hagstrom, 5 NY2d 643, 646, rearg denied 6 NY2d 882; see also, Gomez v New York City Hous. Auth., 217 AD2d 110, 117). It follows, then, that our approval of the introduction of expert testimony bearing upon the machinations of street-level drug-dealing operations to explain the absence of buy money and drugs upon a defendant when arrested where the defendant had been observed in possession of such items at the time of a recently concluded sale, must be premised upon the existence of a factual basis for the opinion typically offered by prosecution experts in such cases; if an expert is to attribute the failure to recover drugs and buy money from a defendant to the workings of a conspiracy, there must be some evidence that there was in fact a conspiracy. Traditionally employed principles of the law of evidence simply do not permit the fact finder to entertain a hypothesis without factual grounding and here the requisite factual fundament for the receipt of the hypothesis proposed by the People’s expert was missing. There was no evidence that the seller in this case acted in concert with anyone to effect the sale with which defendant was charged. Indeed, there was not a scintilla of proof interpretable as indicative of the involvement of a "steerer” or a "stash man” or a "money man” or a "lookout” or a "manager” and we do not think that the involvement of such functionaries may be reliably hypothesized by an a expert or inferred by a jury simply because no drug money or contraband was found on the defendant.
An obvious explanation for the absence of drugs and buy money on an individual arrested for a very recent sale of *21narcotics to an undercover officer who observed such items in the seller’s possession at the time of the transaction is that the wrong person was arrested. Indeed, unless the police are presumed to be infallible in the context of "buy-and-bust” operations—and such a presumption, apart from being highly dubious empirically, would be legally impermissible in the context of a criminal trial—there is no reason to suppose that the inference of misidentification a fact finder might well draw from the prosecution’s own evidence in a case such as the one before us, would, as the People repeatedly suggest, necessarily, or even likely, be attributable to "confusion”. And, in the absence of any factual basis to characterize this entirely rational inference as one born of confusion, we can perceive no permissible role for expert opinion. Expert testimony is admissible only insofar as it may assist the jury to understand the significance of the evidence properly before it when that evidence is not readily interpretable by a layperson (see, People v Taylor, 75 NY2d 277, 288-293; People v Cronin, 60 NY2d 430, 432-433). It is not a means of establishing basic facts not yet in evidence and as to which the expert is not competent to testify (see, Cassano v Hagstrom, supra; O’Shea v Sarro, 106 AD2d 435, 437; Lipsius v White, 91 AD2d 271, 279), or for placing before the jury a wholly speculative explanation to conceal a gap or inconsistency in proof (see, Lipsius v White, supra; Matter of Aetna Cas. & Sur. Co. v Barile, 86 AD2d 362). Accordingly, where, as here, there was no reason to suppose that the significance of such evidence as there was of the transaction at issue was beyond the grasp of the average, juror, expert testimony was not admissible to explain how a conspiracy, if it had existed, might have accounted for the absence of drugs and buy monéy on defendant’s person at the time of his arrest. The mere fact that the jury in performing its traditional function of choosing between such competing inferences as were justified by the evidence might have embraced an inference with exculpatory consequence did not justify the conclusion that the jury was confused and in need of expert guidance. The provision of such "guidance” in a situation where the need of it had not been established constituted a palpable intrusion upon the rightful and exclusively held prerogative of the fact finder to draw conclusions from the evidence (see, People v Cronin, supra, at 432).
The prejudicial consequence of admitting the testimony of the prosecution’s expert—effectively inviting the jury to infer conspiratorial involvement from what otherwise might have *22been perceived as an exculpatory circumstance—cannot be overstated. Indeed, it is highly unlikely that even a meritorious claim of misidentification, as defendant’s may well have been, could have prevailed in the face of a speculative onslaught of the sort to which the expert’s testimony in this case gave license.
Although the receipt of the expert’s testimony constitutes a sufficient ground for reversal, we address a remaining appellate point raised by defendant since it presents an issue that may well arise when defendant is retried.
Contrary to defendant’s contention, we believe the trial court’s partial closure of the courtroom during the testimony of the undercover officer to have been proper. The undercover officer testified at the Hinton hearing that he continued to be involved in buy-and-bust operations in the vicinity of defendant’s arrest, which occurred within 10 blocks of the courthouse. In this connection, the undercover stated that he had only the previous day purchased narcotics in that same area; that he had four or five open cases arising from purchases he had made in that location; and that he would continue to work in that location in the immediate future. This testimony, we think, does not materially differ from that found by the Court of Appeals to justify closure in People v Pearson (82 NY2d 436). There, the undercover officer identified a specific location readily accessible from the courthouse where the defendant had been arrested and where the officer had made numerous other arrests and would continue to work extensively (supra, at 443). Citing these circumstances, the Court of Appeals observed, "We cannot say that the trial court abused its discretion in concluding that * * * testifying in an open courtroom might endanger the undercover officer’s safety” (supra, at 443). Given the similarity of the factual predicate here involved, a different conclusion as to whether the trial court’s discretion respecting courtroom closure had been properly exercised would be anomalous. Nor, given the sufficiency of the predicate for closure of the courtroom during the testimony of the undercover officer in this buy-and-bust case, was it incumbent upon the court sua sponte explicitly to consider alternatives to closure (People v Ayala, 90 NY2d 490; People v Pearson, supra). Moreover, it is more than impliedly clear (see, People v Ayala, supra, at 504) that the trial court in this case did in fact consider alternatives to total closure since the defendant’s family and friends were in the end permitted to remain in the courtroom during the undercover officer’s testimony.
*23Our conclusion that the trial court did not err in closing the courtroom based upon the circumstances as they were at the time of defendant’s first trial upon the instant indictment, however, should not be read to preclude a different resolution of the closure issue when defendant is retried. By that time, of course, the circumstances relevant to closure may well have changed.
Accordingly, the judgment of the Supreme Court, Bronx County (Martin Marcus, J.), rendered September 19, 1994, convicting defendant, after a jury trial, of criminal sale of a controlled substance in the third degree, and sentencing him to an indeterminate prison term of 6 to 12 years, should be reversed, on the law and in the interest of justice, and the matter remanded for a new trial.