R v Reddick, 1991 – 1 SCR 1086 (McLachlin):

[3] The defence of honest but mistaken belief that the victim consented to the sexual intercourse on which the charge of sexual assault is based is available only where it is supported by evidence from sources other than the accused, which lends the defence an “air of reality”. As McIntyre J. put it in Pappajohn v. The Queen, [1980] 2 S.C.R. 120, at p. 133:

To require the putting of the alternative defence of mistaken belief in consent,
there must be, in my opinion, some evidence beyond the mere assertion of belief
in consent by counsel for the appellant. This evidence must appear from or be
supported by sources other than the appellant in order to give it any air of reality.
[Emphasis added.]