Luke, use the source. Looking in hv.c I see the following possible causes for a NULL return:

• passing a NULL HV * arg
• needs_copy && !xhv->xhv_array && !needs_store (no, I don't know what this means)

Notice that only a bad HV * arg will cause a 0 return and running out of memory won't - the program will just croak() with an Out of Memory error.

-sam