Null function parameters

Hi All,

I am trying to create a function that takes an int as its param and insert
the value into a table. The problem occurs when the value passed is NULL,
the error message returned is

- Execute failed ERROR: ExecAppend: Fail to add null value in not null
attribute type

However my understanding was that if the default value is SQL NULL then any
values passed into the function that are null would be treated as 'NULL'.

This doesn't seem to be the case. Chances are I am overlooking something,
could any one point me in the right direction?

Cheers

Graham