Returning NULL from functions

I am working on date/time stuff, and in the spirit of cleaning up
interesting but marginally useful features I've dropped support for
"invalid" for the timestamp and timestamptz types. 

To help with upgrading, I thought that I'd map it to a NULL value, and
see the following in the regression tests:

-- Obsolete special values
INSERT INTO TIMESTAMP_TBL VALUES ('invalid');
ERROR:  OidFunctionCall3: function 1150 returned NULL

Is this error message a feature of all returns of NULL, particular to
input routines, or can I somehow mark routines as being allowed to
return NULL values?

Comments?
                       - Thomas