invalid type internal size -1

I all,
I'm tryng to declare my email type.
I think I have to do it in this way:

create type email ( INPUT = email_in,                    OUTPUT = email_out,                    RECEIVE = email_recv,
                SEND = email_send,                    INTERNALLENGTH = VARIABLE,                    PASSEDBYVALUE = f,
                 ALIGNMENT = int4 );
 


however I get:
ERROR: invalid type internal size -1

In the doc is written:

Base data types can be fixed-length, in which case internallength is a positive integer, or variable 
length, indicated by setting internallength to VARIABLE. (Internally, this is represented by setting 
typlen to -1.)


I'm doing this on Postgres 7.4.1


Am I missing something ?

Regards
Gaetano Mendola