Alin,
I have this declaration that accompanies one of the available udf
libraries:

DECLARE EXTERNAL FUNCTION LPAD
CSTRING(256),
CSTRING(1),
INTEGER
RETURNS CSTRING(256) FREE_IT
ENTRY_POINT 'fudlib_lpad' MODULE_NAME 'fudlib';

It seems to work fine in Firebird 1.5. I hope it works for you.
Could it be the NULL statements in your declaration?

Jack