Calling overloaded function with NULL argument

Hello,

i've found a strange behaviour when you have overloaded functions with 
the same name and different argument types and try to call them with NULL.
The stored procedures with "text" and "text[]" argument type gets called 
- the variants with "integer" / "integer[]" produce the, i think 
expected, behaviour and raise an exception "ERROR:  FUNCTION 
foo(unknown) IS NOT UNIQUE".

Is there an explanation or is this a bug?

I've tested this on PG 8.3.1 and 8.2.4, see 
http://pgsql.privatepaste.com/2bCQg0FEPX

Best regards,
Patryk Kordylewski