Data Access :: Query Apparently Works Illogically - Getting Null Resultset

I have this query in a stored procedure.  I will simplify it so that it shows the issue I am having.

I have two tables.  One table has a PK called PRSubLineID.  the other table has that in a foreign key.

I did a query like this using the IN Statement

SELECT PRSubLineID, PRSUBLINENumber, f2, f3,
FROM PRSUBLINES PRSL
WHERE PRSL.PRSUBLINENumber LIKE '%test%'
AND PRSL.PRSubLineID NOT IN
(SELECT CSL.PRSUBLineID FROM CtrSubLines CSL)

This is supposed to select those items from one table with a PRSubLineNumber containing the work Test which has a primary key that has not been used as a foreign key in the other table. However I am getting an empty resultset back.

But when I comment out the "And" and end the query after '%test%', Like this:

SELECT PRSubLineID, PRSUBLINENumber, f2, f3,
FROM PRSUBLINES PRSL
WHERE PRSL.PRSUBLINENumber LIKE '%test%'

I show a resultset containing three records whose PRSubLineID' s are 2384, 2385, 2386.

But when I add this query to the query window:

Select * FROM CtrSubLines CSL WHERE CSL.PRSubLineID in (2384, 2385, 2386)

I also get a null resultset. So hoping to decipher what was going on I changed the initial query to look like this.

SELECT PRSubLineID, PRSUBLINENumber, f2, f3,
FROM PRSUBLINES PRSL
WHERE PRSL.PRSUBLINENumber LIKE '%test%'
AND PRSL.PRSubLineID /*NOT*/ IN
(SELECT CSL.PRSUBLineID FROM CtrSubLines CSL)

And this still returned a null result set.

Basically this is saying that my PRSubLineIDs are neither included nor excluded in the set of all values of PRSubLineID in the CtrSubLines table. But that is simply impossible.