Rhonda and Keith Thompson appeal following the district court’s entry of judgment in their favor pursuant to Federal Rule of Civil Procedure 68, but adverse denial of their post-judgment motions to amend the judgment and for reconsideration. For the reasons stated, we vacate the judgment and remand the case for further proceedings consistent with this opinion.
In October 2006, the Thompsons filed a lawsuit against Southern Farm Bureau Casualty Insurance Company (Southern Farm), seeking $100,000 in underinsured motorist coverage, plus attorney fees, prejudgment interest, and litigation costs. In January 2007, Southern Farm filed an “Offer of Judgment” stating, as relevant, “Farm Bureau offers to tender its policy limits of $100,000 as an offer of judgment, but denies any liability for costs, penalty, interest or attorney’s fees'.” In response, the Thompsons filed an “Acceptance of Offer of Judgment” which stated that they “accepted] the offer made by defendant in its Offer of Judgment ... by which defendant offered to allow judgment to be taken against it for $100,000.00 pursuant to Rule 68 of the Federal Rules of Civil Procedure.” The clerk of the court then entered judgment against Southern Farm for $100,000 pursuant to Rule 68. Thereafter, Southern Farm tendered to the Thomp-sons a check for $100,000, on which Southern Farm indicated that the payment covered “ANY AND ALL CLAIMS,” and with which Southern Farm imposed certain conditions. The Thompsons refused to accept the check as tendered and with Southern Farm’s conditions. The Thomp-sons also filed a motion in the district court for statutory penalties, attorney fees, and prejudgment interest, which the court construed as a motion to amend the judgment.
At a subsequent hearing, the district court found that Southern Farm’s offer was unambiguous: it “offered to settle the case essentially by payment of $100,000, and not paying penalty, interest, and attorney’s fees.” The court thus denied the Thompsons’ motion. The Thompsons then filed a motion to reconsider, which the court also denied. On appeal, the Thomp-sons argue (1) the district court erred in concluding Southern Farm’s offer of judgment unambiguously excluded costs and other remedies, and (2) if the court’s interpretation was correct, then the offer was void as a matter of law because an offer of *904judgment under Rule 68 may not exclude costs.
We agree with the district court’s conclusion that Southern Farm’s offer of judgment unambiguously excluded costs, because the evidence as a whole unequivocally demonstrated that the offer meant that Southern Farm would not be liable for costs or any other remedies upon the Thompsons’ acceptance of the offer. Cf. Stewart v. Prof'l Computer Ctrs., 148 F.3d 937, 938-40 (8th Cir.1998) (Rule 68 offer providing that “judgment be entered on any or all counts against Defendant in a total amount not to exceed FOUR THOUSAND FIVE HUNDRED AND No/100 DOLLARS ($4,500.00) as provided in Rule 68” was open to more than one interpretation; although defendant intended offer to include attorney fees, and plaintiff had requested such fees among counts in complaint, plaintiff interpreted offer to permit her to seek fees in addition to lump-sum offer; because parties did not manifest mutual assent to same terms, court vacated judgment on ground that there was no valid offer and acceptance under Rule 68); Radecki v. Amoco Oil Co., 858 F.2d 397, 399-403 (8th Cir.1988) (reversing entry of judgment made pursuant to Rule 68 because no binding agreement resulted when plaintiffs purported acceptance of defendant’s Rule 68 offer did not mirror terms of offer, and parties thus did not manifest mutual assent to terms; relying upon extrinsic evidence to interpret original offer).
However, we conclude that the district court erred in determining that Southern Farm’s offer of judgment was legally valid. “[P]rinciples of contract law are applied to test whether there has been valid offer and acceptance under Rule 68.” Stewart, 148 F.3d at 939; see Marek v. Chesny, 473 U.S. 1, 5, 105 S.Ct. 3012, 87 L.Ed.2d 1 (1985) (in analyzing denial of costs incurred after offer of judgment, court must first determine whether offer was valid under Rule 68). Although a Rule 68 offer of judgment may offer a lump-sum payment that includes costs, such an offer may not exclude costs. See id. at 6, 105 S.Ct. 3012 (“As long as the offer does not implicitly or explicitly provide that the judgment not include costs, a timely offer will be valid.”); see also Util. Automation 2000, Inc. v. Choctawhatchee Elec. Coop., Inc., 298 F.3d 1238, 1241 (11th Cir.2002) (offer of judgment is proper as long as it does not explicitly exclude costs; “The sole constraint Rule 68 places on offers of judgment is its mandate that an offer include ‘costs then accrued.’ ”). While it is true that some circuits have allowed parties to define which costs are included in the judgment, see id. at 1249, Marek forbids a party from excluding all costs in its offer, which Southern Farm attempted to do. We thus hold that there was no valid offer under Rule 68, and that the judgment entered pursuant to Rule 68 is void.1 Accordingly, we vacate the judg*905ment and remand the case to the district court for further proceedings consistent with this opinion.
. The dissent argues that even if the offer of judgment was an invalid Rule 68 offer, it nonetheless was a valid settlement offer. The parties, however, including Southern Farm, treated the offer as an offer of judgment under Rule 68 rather than as a simple settlement offer. Southern Farm titled its pleading, "Offer of Judgment,” consistent with the language of Rule 68, and filed it with the district court rather than simply presenting it as a proposed consent judgment to the Thompsons during the course of settlement negotiations. The Thompsons explicitly accepted the offer "pursuant to Rule 68,” and the court entered judgment pursuant to Rule 68. The district court also consistently identified it as a Rule 68 offer in its subsequent orders. Southern Farm never objected to this characterization and does not argue on appeal that the offer was anything other than a Rule 68 offer. Indeed, we have little doubt that Southern Farm filed its Offer of Judgment with the district court in order to avoid additional costs pursuant to Rule 68(d). Because it has *905all the characteristics of a Rule 68 offer of judgment, and the parties and the court consistently treated it as such, we disagree with the dissent and conclude that Southern Farm’s offer was not a simple settlement offer that the Thompsons accepted. See Util. Automation 2000, 298 F.3d at 1244 ("While an offeree can respond to an ordinary settlement offer through a counteroffer or seek to clarify or modify its terms, a Rule 68 offeree is at the mercy of the offeror’s choice of language and willingness to conform it to the understanding of both parties.”).
Even if we were to consider the offer of judgment as an invalid Rule 68 offer but a valid settlement offer, the Thompsons explicitly accepted the offer "pursuant to Rule 68.” Because Marek requires a Rule 68 offer to include costs, the Thompsons’ acceptance "pursuant to Rule 68” differed from Southern Farm's offer that excluded costs. Therefore, the parties did not manifest mutual assent to the same terms. See Stewart, 148 F.3d at 940; Radecki, 858 F.2d at 403.