GPF on trying to assign value from call return

[Josh Beitler]

syscall_t func = (syscall_t)location;
func(r->ebx, r->ecx, r->edx, r->esi, r->edi); // works
uint32_t ret = func(r->ebx, r->ecx, r->edx, r->esi, r->edi); // doesn't work