The Equitable Defense of Non Est Factum
The equitable principle of non est factum, Latin for “it is not [my] deed”, is a defense available to someone who has been misled into executing a deed or signing a document which is fundamentally different from that which he intended to execute or sign.[1] It is part of a “special category of the law of mistake and is extremely narrow in scope”.[2] The mistake must have been as to the essential nature of the transaction, rather than as to its terms, “a mistake as to the legal effect of those terms by the signatory or by his legal adviser will not suffice”.
The Supreme Court of Canada’s decision of Marvco Color Research Ltd. v. Harris (“Marvco”),[3] is the governing authority in Canada on non est factum.
In Marvco, the question before the Court was which of two parties was to bear the burden of the fraud of a third. The Court concluded that as between an “innocent” third party and one who is careless in the execution of a document, the careless party should bear any loss resulting from his or her own carelessness. In that case, the respondents signed a security agreement, in favor of the appellants, securing the performance of another transaction. The respondents did not read the document. They relied upon misrepresentations as to its nature made to them by one of the covenantors whose performance was being guaranteed by the document in question. Relying on that document, the appellants released one of the covenantors from liability under a mortgage he had given to them.[4]
Following the decision in Marvco, the principles of non est factum can be summarized as follows:
- The burden of proving non est factum rests with the party seeking to disown their signature. For a person of full capacity, the application of the doctrine must be kept within narrowly prescribed limits;
- The person who seeks to invoke the remedy must show that the document signed is fundamentally different from what the person believed he or she was signing; and
- Even if the person shows such a fundamental difference, the court must examine whether the signer was careless in failing to take reasonable precautions in the execution of the document. The court must also consider the conduct of the party relying on the document and whether they qualify as an innocent party, in order to determine which party, by application of reasonable care, was in the better position to avoid the loss.[5]
Recent application of non est factum
In the recent decision of Chieffallo v. Blair (“Chieffallo”),[6] the respondent sought to use the defence of non est factum. In Chieffallo, the deceased executed a Last Will and Testament, appointing two of her children, Eisa Chieffallo (the “Applicant”) and Michael Blair (the “Respondent”) as co-estate trustees. The Will afforded great discretion on the estate trustees to distribute the assets of the estate, specifically “with the view to the best interests of [the Deceased’s] family, in their absolute and unfettered discretion deem fit and appropriate”.[7]
The deceased passed away on December 26, 2021. On or about March 23, 2022, the Respondent executed a renunciation of his right to apply for a certificate of appointment of estate trustee. Within days, the Respondent sought to retract his renunciation. The Applicant rejected this argument, noting that as a result he was no longer estate trustee.[8]
The Respondent claimed he did not understand the purpose and consequence of the renunciation and was otherwise misled, which the court took to be a plea of non est factum. Applying the principles elucidated in Marvco, the court rejected the Respondent’s claim and affirmed the Applicant as the estate trustee. In his reasoning, Justice Bell noted that:
I reject Michael’s claim that he did not understand what he was signing. The document is short (one-page) and clear. The document’s title is, “Renunciation of Right to A Certificate of Appointment of Estate Trustee (or Succeeding Estate Trustee) with a Will.” The title is repeated in the document itself, together with the sentence, “I renounce my right to a certificate of appointment of estate trustee (or succeeding estate trustee) with a will.” If Michael did not understand the renunciation, he was provided the opportunity to consult with a lawyer. He chose not to.[9]
Concluding Comments
In summary, the equitable defence of non est factum can be used when one enters into a deed or signs a document which is fundamentally different from that which they intended to execute or sign. The analysis is highly fact specific,[10] narrow in scope, and does not assist those who carelessly or blindly sign any document.[11]
—
[1] Andrews & Millet, Law of Guarantees, (4th ed) (London: Sweet & Maxwell, 2001) at 104.
[2] Ibid.
[3] 1982 CanLII 63 (SCC).
[4] Farrell Estates Ltd. v. Win-Up Restaurant Ltd., 2010 BCSC 1752 at para. 85.
[5] Ibid., at para. 100.
[6] Chieffallo v. Blair, 2025 ONSC 3411 (CanLII).
[7] Ibid. at paras 1, 2, 3, 4 and 7.
[8] Ibid. at para 5.
[9] Ibid. at paras 25 and 26.
[10] Marvco at para 27.
[11] General Accident Assurance Co. of Canada v. Singh, 2000 CarswellOnt 5424, [2000] O.F.S.C.I.D. No. 141 at para 13.
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Written by: Oliver O'Brien
Posted on: September 17, 2025
Categories: Commentary
The equitable principle of non est factum, Latin for “it is not [my] deed”, is a defense available to someone who has been misled into executing a deed or signing a document which is fundamentally different from that which he intended to execute or sign.[1] It is part of a “special category of the law of mistake and is extremely narrow in scope”.[2] The mistake must have been as to the essential nature of the transaction, rather than as to its terms, “a mistake as to the legal effect of those terms by the signatory or by his legal adviser will not suffice”.
The Supreme Court of Canada’s decision of Marvco Color Research Ltd. v. Harris (“Marvco”),[3] is the governing authority in Canada on non est factum.
In Marvco, the question before the Court was which of two parties was to bear the burden of the fraud of a third. The Court concluded that as between an “innocent” third party and one who is careless in the execution of a document, the careless party should bear any loss resulting from his or her own carelessness. In that case, the respondents signed a security agreement, in favor of the appellants, securing the performance of another transaction. The respondents did not read the document. They relied upon misrepresentations as to its nature made to them by one of the covenantors whose performance was being guaranteed by the document in question. Relying on that document, the appellants released one of the covenantors from liability under a mortgage he had given to them.[4]
Following the decision in Marvco, the principles of non est factum can be summarized as follows:
Recent application of non est factum
In the recent decision of Chieffallo v. Blair (“Chieffallo”),[6] the respondent sought to use the defence of non est factum. In Chieffallo, the deceased executed a Last Will and Testament, appointing two of her children, Eisa Chieffallo (the “Applicant”) and Michael Blair (the “Respondent”) as co-estate trustees. The Will afforded great discretion on the estate trustees to distribute the assets of the estate, specifically “with the view to the best interests of [the Deceased’s] family, in their absolute and unfettered discretion deem fit and appropriate”.[7]
The deceased passed away on December 26, 2021. On or about March 23, 2022, the Respondent executed a renunciation of his right to apply for a certificate of appointment of estate trustee. Within days, the Respondent sought to retract his renunciation. The Applicant rejected this argument, noting that as a result he was no longer estate trustee.[8]
The Respondent claimed he did not understand the purpose and consequence of the renunciation and was otherwise misled, which the court took to be a plea of non est factum. Applying the principles elucidated in Marvco, the court rejected the Respondent’s claim and affirmed the Applicant as the estate trustee. In his reasoning, Justice Bell noted that:
I reject Michael’s claim that he did not understand what he was signing. The document is short (one-page) and clear. The document’s title is, “Renunciation of Right to A Certificate of Appointment of Estate Trustee (or Succeeding Estate Trustee) with a Will.” The title is repeated in the document itself, together with the sentence, “I renounce my right to a certificate of appointment of estate trustee (or succeeding estate trustee) with a will.” If Michael did not understand the renunciation, he was provided the opportunity to consult with a lawyer. He chose not to.[9]
Concluding Comments
In summary, the equitable defence of non est factum can be used when one enters into a deed or signs a document which is fundamentally different from that which they intended to execute or sign. The analysis is highly fact specific,[10] narrow in scope, and does not assist those who carelessly or blindly sign any document.[11]
—
[1] Andrews & Millet, Law of Guarantees, (4th ed) (London: Sweet & Maxwell, 2001) at 104.
[2] Ibid.
[3] 1982 CanLII 63 (SCC).
[4] Farrell Estates Ltd. v. Win-Up Restaurant Ltd., 2010 BCSC 1752 at para. 85.
[5] Ibid., at para. 100.
[6] Chieffallo v. Blair, 2025 ONSC 3411 (CanLII).
[7] Ibid. at paras 1, 2, 3, 4 and 7.
[8] Ibid. at para 5.
[9] Ibid. at paras 25 and 26.
[10] Marvco at para 27.
[11] General Accident Assurance Co. of Canada v. Singh, 2000 CarswellOnt 5424, [2000] O.F.S.C.I.D. No. 141 at para 13.
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