Wells Fargo Bank, N.A. (Wells Fargo) appeals from the district court’s award of summary judgment in favor of Joel and Tanya Karnitz, who brought this action seeking a declaration that the mortgage held by Wells Fargo on their residence is invalid under Minnesota Statute § 507.02. We agree with Wells Fargo that given the undisputed facts of this case, the Karnitzes should be estopped from relying on § 507.02 to challenge the validity of the mortgage. We therefore reverse the district court’s judgment and remand with instructions to enter judgment in favor of Wells Fargo.
I.
In 2001, the Karnitzes built their home on land they purchased in LaPorte, Minnesota. They financed construction of the house through a short-term construction loan from Centennial National Bank. Both Joel and Tanya signed the loan documents and a mortgage in favor of Centennial. Upon completion of the house, the Karnitzes sought to pay off the construction loan with a traditional 30-year mortgage loan, which Centennial arranged through Wells Fargo. Joel signed a note for $136,800 payable to Wells Fargo and executed a mortgage on the property in favor of Wells Fargo. The Karnitzes used the Wells Fargo loan proceeds to pay off the construction loan, thereby securing a release of the Centennial mortgage.
Tanya did not sign the Wells Fargo loan documents or the new mortgage because Wells Fargo never asked her to sign any of the documents. Tanya testified in her deposition that she knew Joel was seeking a loan and mortgage from Wells Fargo to pay off the Centennial construction loan; that she knew the loan would result in a mortgage in favor of Wells Fargo; that she approved of Joel obtaining the loan and granting the mortgage to Wells Fargo; and that she wanted to obtain the loan in exchange for the mortgage.
On April 17, 2005, the Karnitzes filed for bankruptcy protection under Chapter 7 of the United States Bankruptcy Code. They listed their house as secured by a first *574mortgage in favor of Wells Fargo and listed the Wells Fargo loan as a joint obligation. The Karnitzes received a discharge in bankruptcy on August 8, 2005. Shortly thereafter, they fell behind on their mortgage payments to Wells Fargo. Wells Fargo initiated foreclosure proceedings and foreclosed on the house on January 17, 2007.
In the meantime, Joel learned that Minnesota law requires a mortgage on a married couple’s homestead to be signed by both spouses under Minnesota Statute § 507.02. The Karnitzes took the position during the foreclosure proceedings that the mortgage in favor of Wells Fargo was invalid because Tanya had not signed it. They brought this action in Minnesota state court seeking a declaration to that effect, and Wells Fargo removed the case to federal court based on diversity jurisdiction. The district court concluded that the unambiguous language of § 507.02 required Tanya’s signature on the mortgage, and without her signature, the mortgage was void ab initio. The district court rejected Wells Fargo’s estoppel arguments and granted summary judgment in favor of the Karnitzes.
II.
We review de novo both the district court’s grant of summary judgment as well as its interpretation of state law. MSK EyEs Ltd. v. Wells Fargo Bank, 546 F.3d 533, 540 (8th Cir.2008). The parties agree that the underlying facts are undisputed. Under Minnesota law, “[t]he application of equitable estoppel presents a question of law.” Minnesota v. Ramirez, 597 N.W.2d 575, 577 (Minn.Ct.App.1999).
With exceptions not here relevant, § 507.02 of the Minnesota Statutes provides: “If the owner is married, no conveyance of the homestead ... shall be valid without the signatures of both spouses.” A conveyance under § 507.02 includes a mortgage. See Minn.Stat. § 507.01. A conveyance that fails to meet these statutory requirements is void and cannot be ratified. See Dvorak v. Maring, 285 N.W.2d 675, 677 (Minn.1979) (explaining that “there cannot be a ratification of a contract for the sale of a homestead that is void due to the lack of a spouse’s signature” because of the interplay with Minn. Stat. § 519.06 concerning when a spouse can and cannot act as the other spouse’s agent).
Despite the plain and unequivocal language of the statute, the Minnesota Supreme Court has “recognized that, even though great importance is attached to the homestead right, under certain circumstances a party may be estopped from denying a sale of the homestead even if the statutory requirements are not met.” Id. The purpose behind the statute is to “ ‘ensure] a secure homestead for families,’ ” Wells Fargo Home Mortg., Inc. v. Newton, 646 N.W.2d 888, 895 (Minn.Ct.App.2002) (quoting Dvorak, 285 N.W.2d at 677), and to.protect against “the alienation of the homestead without the willing signature of both spouses,” Dvorak, 285 N.W.2d at 678. In certain circumstances when the purpose of the statute is not at risk, the Minnesota courts have applied estoppel to prevent a party from challenging the validity of a conveyance of a homestead. In its most recent discussion of the equitable estoppel doctrine in this specific context, the Minnesota Supreme Court stated, in addressing whether a nonsigning spouse should be estopped from asserting the protections of § 507.02 to void a conveyance by her spouse, that estoppel applies where (1) the nonsigning spouse consents to and has pri- or knowledge of the transaction, (2) the nonsigning spouse retains the benefits of the transaction, and (3) the party seeking to invoke estoppel has sufficiently changed *575its position to invoke the equities of estoppel. See Dvorak, 285 N.W.2d at 677-78 (discussing Seitz v. Sitze, 215 Minn. 452, 10 N.W.2d 426 (1943); Fuller v. Johnson, 139 Minn. 110, 165 N.W. 874 (1917)). All three factors must be present, cf. Anderson v. First Nat’l Bank of Pine City, 303 Minn. 408, 228 N.W.2d 257, 260 (1975) (estoppel not appropriate where husband did not learn that wife forged his signature until after the transaction was complete (such that the first factor was not met), even though he did retain the benefits of the mortgage), and the third factor is critical, Dvorak, 285 N.W.2d at 678 (“[Detrimental reliance by the party seeking relief is critical to a finding of estoppel.”).
Although the Minnesota Supreme Court refused to apply estoppel to the facts of the case in Dvorak because the purported transferee would not be substantially prejudiced by voiding the conveyance, see id. (noting that the only performance on the buyers’ part was payment of $1,000 in earnest money, which was returned; the buyers had not sold their own home or taken possession of the disputed property), we read Dvorak as definitive authority on the factors necessary to apply equitable estoppel under Minnesota law to preclude a party from relying on § 507.02 to avoid a conveyance. See also Bullock v. Miley, 133 Minn. 261, 158 N.W. 244, 245 (1916) (applying estoppel where both spouses intended to alienate the homestead and gave expression to that intent and the party to whom the improper conveyance was made acted in reliance on the validity of the conveyance such that the denial of its validity four years later would work a manifest injustice to him).
Applying the equitable estoppel factors articulated in Dvorak, it is undisputed that Tanya knew of and intended to mortgage the homestead to Wells Fargo prior to its execution; she retained the benefit of that mortgage by using the proceeds to pay off the construction loan (which she had signed and was obligated to repay) and to obtain a release of the construction loan’s accompanying mortgage; and Wells Fargo significantly changed its position in reliance on the validity of its mortgage by lending over $130,000 in exchange for a lien on the property. The Karnitzes do not dispute the existence of these facts. Further, they did not dispute the validity of the mortgage until four years after it was executed and they were facing foreclosure because they could not keep up with the obligations of the accompanying note. Under these facts, the Karnitzes should be estopped from now claiming that the mortgage is void in order to keep their home, on which they both intended to grant Wells Fargo a valid mortgage, without paying for it.1 Strict compliance with the statute in these circumstances does not further the policy behind the statute; rather, it flaunts it by converting what the Legislature intended as a shield into a sword. See Dvorak, 285 N.W.2d at 677 n. 3 (“ ‘The doctrine [of equitable estoppel] is not invoked to render valid a contract which is void under ... statutes for the benefit and protection of the homestead claimants, but it is invoked to prevent the successful perpetration of fraud by preventing wrongdoers from urging the provisions of such statutes to shield them in their tortious conduct.’ ”) (quoting Engholm v. Ekrem, 18 N.D. 185, 119 N.W. 35, 38 (1908)).
The district court in this case recognized that a party may be estopped from asserting the invalidity of the conveyance based *576on the lack of a spouse’s signature, but rejected the estoppel defense based on a line of Minnesota cases holding that “an oral promise to give a mortgage on a couple’s homestead by one spouse does not give rise to estoppel.” (Add. at 5) (citing Kingery v. Kingery, 185 Minn. 467, 241 N.W. 583, 584-85 (1932); Butler Bros. Co. v. Levin, 166 Minn. 158, 207 N.W. 315, 316 (1926)). The district court’s reliance on this line of cases is misplaced. The point of the holding in Butler Bros, was that estoppel could not be based on a promise of future conduct. In that case, a bank tried to foreclose on a homestead where only the husband signed the mortgage. The wife promised to make sure the husband’s debt would be paid, and, if it was not, she would then sign the mortgage. Butler Bros., 207 N.W. at 316. When the wife failed to make good on the debt and still refused to sign the mortgage, the creditor argued estoppel based on its agreement to stop collections against the husband in detrimental reliance on the wife’s promise. The court rejected the estoppel argument because the wife’s promise “related wholly to the intention of the promisor with respect to future action. It did not relate to present or past conditions or rights. The law is that a promise, representation, or concealment, in order to constitute an estoppel, even when relied upon, must ‘have reference to a present or past state of things.’ ” Id. (emphasis added) (quoting Bigelow on Estoppel, 636). The case at bar does not involve a promise at all, oral or otherwise. Further, there is no indication in Butler Bros, that the wife had prior knowledge or had otherwise acquiesced in the contested mortgage that lacked her signature. In fact, the court noted that estoppel may be held against a nonsigning spouse in the case of a homestead, distinguishing Osman v. Wisted, 78 Minn. 295, 80 N.W. 1127 (1899), on the basis that estoppel was proper there where
the wife acquiesced in and ratified the designation of the homestead. Butler Bros., 207 N.W. at 316. The district court here erred in focusing on the effect of a nonexistent oral promise without considering the factors described in Dvorak.
We recognize that the doctrine of equitable estoppel generally involves some type of misrepresentation or at least negligent culpability on the part of the person against whom it is claimed. See, e.g., Birch Publ’ns, Inc. v. RMZ of St. Cloud, Inc., 683 N.W.2d 869, 873-74 (Minn.Ct. App.2004) (describing equitable estoppel as arising “when ‘one by his acts or representations, or by his silence when he ought to speak, intentionally or through culpable negligence, induces another to believe certain facts to exist, and such other rightfully acts on the belief so induced’ ” such that he is prejudiced (quoting Transam. Ins. Group v. Paul, 267 N.W.2d 180,183 (Minn. 1978))). Despite this general statement of the equitable estoppel doctrine, the Minnesota cases that apply estoppel in the specific context of the homestead signature requirement of § 507.02 have found such a “culpability” requirement satisfied by the nonsigning spouse’s prior knowledge and agreement of the conveyance coupled with the retention of the benefits of the conveyance. For example, in Seitz, a son took care of his mother for several years in exchange for his father’s promise that he would receive their homestead upon their deaths. When the father’s other heirs challenged the conveyance to the son for failure to comply with the homestead signature requirement, the Minnesota Supreme Court used the doctrine of equitable estoppel to hold that § 507.02 “cannot now be invoked after full performance of the contract by [the son] and after acceptance of the benefits of such performance by his parents with full knowledge of the agreement.” Seitz, 10 N.W.2d at 429. In Bull*577ock, a husband was estopped from claiming title to the homestead he had sold where the original conveyance lacked his wife’s signature. The Minnesota Supreme Court applied estoppel because “both plaintiff and his wife intended to convey the homestead ... and believed that they had done so; [ ] no offer to return the consideration received for the homestead or any part thereof has ever been made; and [ ] plaintiff is seeking to recover the homestead without returning any part of such consideration.” Bullock, 158 N.W. at 245. The Minnesota Supreme Court applied estoppel in these cases despite the absence of any type of misrepresentation or inducement.
The facts of Bullock are strikingly similar to the facts here. Both of the Karnitzes intended to convey a mortgage to Wells Fargo; they have made no offer to return the consideration received; and they now seek-several years later-to retain their homestead without paying for it. Fulfilling our duty to apply state law as we believe the highest court of the state would apply it, see Leonard v. Dorsey & Whitney LLP, 553 F.3d 609, 612 (8th Cir.2009), we are bound to follow the Minnesota Supreme Court cases most directly on point, particularly Seitz, Bullock, and Dvorak. Equitable estoppel is appropriate under the facts of this case, and the district court erred in concluding otherwise.
III.
The district court’s judgment granting summary judgment to the Karnitzes is reversed, and the case is remanded for entry of summary judgment in favor of Wells Fargo.
. According to the parties, if the mortgage is invalid, the resulting unsecured note to Wells Fargo for the balance owed on the house would be discharged as part of a reopened bankruptcy estate.