It's doing exactly what it's supposed to do.

Either re-write the code and make duplicate copies of fnAdd, one for non-isr and
one for isr operation.

Or use the isr level-setting pragma iff you cannot possibly have a call to fnAdd
while fnAdd is already running.

This is because the PIC doesn't have a stack.  All the RAM allocations are
performed by the compiler.

The poor little processor just isn't a PC <G>

Andy