XS callback function

I've written a callback function for an XS module designed to call a 
perl function when invoked.

If I call the function directly, from within the C code, it runs through 
completely as expected.

If its invoked as a callback from the library I am wrapping, it does 
immediately at "dSP;" i.e. SV **sp = PL_stack_sp;

Specifically PL_stack_sp seems to cause SEGV when touched - I can't even 
see the value, printf dies also.

I can't find anything similar discussed anywhere, does anyone have an 
idea what might be happening?