WHERE pk1 AND pk2 returns 0 records

[Brian Shearer]

I have a simple query:
SELECT * FROM cart 
WHERE CartID=999 AND ProdID=333

The primary key is made of both CartID and ProdID, both integers.

The query will not return the record in question

This returns the row:
SELECT * FROM cart 
WHERE CartID=999

And this returns the same row:
SELECT * FROM cart 
WHERE ProdID=333

But this returns zero records:
SELECT * FROM cart 
WHERE CartID=999 AND ProdID=333

Any ideas? 

Brian