unclear enum error messages

There are a few error messages like this in the code:
       /*        * We rely on being able to get the specific enum type from the calling        * expression tree.  The
generictype mechanism should have ensured that        * both are of the same type.        */       enumtypoid =
get_fn_expr_argtype(fcinfo->flinfo,0);       if (enumtypoid == InvalidOid)               ereport(ERROR,
             (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),                                errmsg("could not determine actual
enumtype")));
 

First, isn't this an internal error?

What is an actual enum type?  And how should a user react if he got this
message?  I would ask, "why not?".

-- 
Peter Eisentraut
http://developer.postgresql.org/~petere/