STM32F042 CAN POLLING WORKS - INTERRUPTS NOT HAPPENING

[GBats]

I have coded it both ways, so the hardware and software works except when switching to interrupt-driven receive. No interrupts happen, though I have checked and rechecked. The global NVIC can interrupt setting is there, and the enable. The CAN priority is set to 1 to allow the SysTick to work for timeouts. The USB interrupt priority is set to 2. I am using all of the snippets in the CAN migration guide from V1 HAL to V2. The transmit always works in polling mode, but I need the receive in interrupt mode. Are there any known quirks about using CAN interrupts on this MPU? Whatever it is seems to be very obscure.