In November, 1975, the defendant was tried in the Superior Court on separate indictments charging him with violations of G. L. c. 265, §§ 171 and ISA.2 The *203defendant did not testify. During the course of their deliberations the jury sent in the following question: "Can the defendant be found guilty of robbery and assault — and not guilty of carrying a pistol?” Near the conclusion of the trial judge’s response to that question he said: "[Y]ou apparently are disturbed about the presence or non-presence of a pistol. There is direct evidence here that there was a pistol and a knife. You don’t have to believe it. But you can't supply the additional fact that there was no weapon unless you find some other sound basis in the evidence which warrants you in drawing that inference” (emphasis supplied). The defendant’s trial counsel took no exception. The defendant was convicted and sentenced on both indictments, and he did not seek appellate review of either conviction.

In April, 1977, the defendant’s present counsel filed a motion for a new trial on both indictments on the ground that the italicized portion of the above quoted instruction had "improperly reversed the burden of proof, requiring [the defendant] to produce evidence of his innocence.” See In re Winship, 397 U.S. 358, 364 (1970); Mullaney v. Wilbur, 421 U.S. 684, 701-702 (1975); Hankerson v. North Carolina, 432 U.S. 233, 240-244 (1977); Commonwealth v. Rodriguez, 370 Mass. 684, 687-689 (1976); Commonwealth v. Johnson, 372 Mass. 185, 192 (1977); Commonwealth v. Greene, 372 Mass. 517, 518-519 (1977); Commonwealth v. Stokes, 374 Mass. 583, 590 (1978); Commonwealth v. Collins, 374 Mass. 596, 599 (1978). The trial judge having retired, the motion was heard by a different judge, who read the trial transcript and ruled adversely to the defendant on the question raised by the motion. That question is now before us on the defendant’s appeal (G. L. c. 278, § § 33A-33G) from the order denying the motion. Commonwealth v. McGrath, 361 Mass. 431, 435 n.2 (1972). Commonwealth v. Thompson, 362 Mass. 382, 384 n.2 (1972). Commonwealth v. Flemmi, 2 Mass. App. Ct. 533, 535 (1974).

*204We have carefully reviewed the italicized instruction in light of the exemplary original charge and all the other instructions given the jury on the subject of the Commonwealth’s burden of proving all the elements of the of-fences charged. See Commonwealth v. Stokes, 374 Mass. at 590-591, and cases cited. It is clear to us that the challenged instruction was unintentional. But a finding that the defendant had been armed with a dangerous weapon when he had entered the victim’s apartment was essential to a conviction under G. L. c. 265, § 18A, and a finding that the defendant was so armed was essential to a conviction on so much of the indictment under G. L. c. 265, § 17, as alleged the commission of the aggravated offence of armed robbery. Virtually the last thing the jury heard was an instruction which, in effect, cast the burden on the defendant of proving that he had not been armed with a dangerous weapon. As counsel may have surmised from our remarks during the argument, we are unable to conclude that the error was not prejudicial to the rights of the defendant.

Accordingly, there must be a new trial on both indictments.

So ordered.