The most obvious answer would be that "assert.h" is not being included or is not being found in your include path. Another explanation is that the assert macro has been undefined at some point after the header was included.
Edit: Since you say that assert.h is included, and we'll assume for the moment that it's being found since it's a standard header, then that leaves us with the last possibility I stated above i.e. that the macro has been undefined.
Since cpool.h itself will not be doing this it must be the case that assert.h is included earlier either by yourself or indirectly by another 3rd party header and the undefining happening between this and your inclusion of cpool.h. This can easily be tested by moving your cpool.h include to the top of your file.