Can a sentencing judge be called as a witness?

In the course of the hearing below, appellant called the sentencing judge who also presided at the hearing on the instant motion as a witness. The calling of the judge who sentenced appellant and who was presiding over the hearing on appellant's motion was, contends appellant, proper; citing Davis v. United States, 8 Cir. 1954, 210 F.2d 118, as authority. It is not in point; and does not so hold. But, urges appellant, it was error for the judge, once called (and to whom appellant contends he addressed a proper question² which required an answer) to perform the dual function of witness and judge, as he was when he sustained appellee's objection to the question addressed to him by appellant.