Problems with UNION-query

I have a sql statement with 2 SELECT statements combined with the keyword UNION. Something like this

SELECT a, b
FROM ... WHERE ...
UNION
SELECT c, d
FROM ... WHERE ...

If i execute this in the Query Analizer everything works fine and i get the results. But if i execute this through Visual Basic (with database.OpenRecordSet(sql) ) i get following error message:

The Microsoft Jet database engine cannot find the input table or query ... Make sure it exists and that its name is spelled correctly.

If i execute the first part of this combined query in VB (e.g. SELECT a, b FROM ... WHERE ...), no errors occur. The same with the second part.

I find it really strange and any hints and tips are welcome!
thanks in advance.Make sure you specify dbname..table. A different database may be your default via odbc or whatever you are using to connect through VB.|||Thx for the suggestion, but i already tried that one and unfortunately it doesn't solve the problem

Someone other hints, tips, tricks,...?sql