Returning null from Userdefined C function

I'm trying to write a function that takes a text input and returns
a text output. I can get this to work. The problem is that I want
to return NULL when the input string doesn't match the criteria I
desire. Unfortunately, returning NULL seems to crash the backend.
i.e. if I did

#include "postgres.h"
text * andytest ( text * str )
{
return NULL;
}

The backend would quit unexpectantly when I ran
select andytest('fds');
or select andytest(NULL);

Obviously, there must be some way to create a NULL text * return
variable, but I haven't been able to find it. I've looked at all
the code I've been able to find to no avail.

-Andy