Ab Initio: Null keys in lookup file

NULL-valued arguments in functions

Most built-in functions return NULL when any of their arguments is NULL. Exceptions are:

Function category	How the functions handle NULL values
Lookup functions	When passed a NULL argument to use as a key or subkey, these functions try to match the NULL value to the corresponding key or subkey of a record in the lookup file.
Certain inquiry and error functions	These functions never return NULL, because they are specifically designed to return indicators of NULL or invalid values.

In Short : If there are NULL values in the fields of a lookup file and NULL values in the parameters passed to a lookup function, the values will match and the function will return a record.