You cannot record your changes: filter issue

[Utreg]

Hi,

I've created a linked (External data to .txt files) database and want to filter out the non nummeric and NULL values/lines.
A query refers to the linked table and have set 1 Criteria to filter them.

If I try the following SQL statement:

SELECT [Tbl user].iUserId, [Tbl user].nvcLastName
FROM [Tbl user]
WHERE ((([Tbl user].iUserId) Is Not Null));

or try:

WHERE (([Tbl user].iUserId)>0);

and switch to data sheet view it runs fine but after I save the Query and open it again the following error message pops up:
You cannot record your changes because a value you entered violates the settings for this table or list..

Can you spot what went wrong?
Thanks.