Judgment, Supreme Court, New York County (Herman Cahn, J.), entered July 13, 1991, which, after a jury trial, rendered judgment for defendant, unanimously reversed, and a new trial ordered, with costs.
In this action for breach of contract and violation of trade secrets, the IAS court erroneously charged the jury in a way that left a clear impression that the burden of proof was on the plaintiff that he, and not a successor corporation, was the real party in interest. The defendant bears the burden of proof on an affirmative defense (see, e.g., Kramer v City of New York, 173 AD2d 155, lv denied 78 NY2d 857), including one *291that asserts that the plaintiff is not the real party in interest (Cleary v Dykeman, 162 App Div 897). The erroneous charge on the burden of proof warrants a new trial (see, Wilson v Nationwide Mut. Ins. Co., 168 AD2d 912, lv dismissed 77 NY2d 940).
In view of the foregoing we need not reach plaintiff’s other arguments. Concur—Carro, J. P., Rosenberger, Ellerin, Kupferman and Ross, JJ.