C function and enum types parameters

I use a C function in my database from 2002, all goes well also with
postgresql 8.3 but with 8.3.1 it no longer works, the problem is with
this line:

text *hibrid = (PG_ARGISNULL( 0) ||
VARSIZE(PG_GETARG_TEXT_P( 0))==VARHDRSZ ?NULL:PG_GETARG_TEXT_P(0));

the argument number 0 is an enum defined by

CREATE TYPE hibridationLevel AS ENUM('none','genus','specie');

the error is "ERROR:  invalid memory alloc request size 2298488997"

I can't figure by now how to correct my error, can anyone help me?

thank you
edoardo
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