NIOS II hard fault due to pulse interrupt

Hi,

I have a NIOS II system and I was sending a pulse interrupt (of 16 clock cycles). The NIOS was going to an exception handler instead of the ISR. I changed the pulse to a level signal and added a clear signal controlled by a PIO bit and now the ISR is executed.

The question is, why the hard fault? I would expect that the NIOS would miss the pulse and do nothing.

Regards,

Carlos