JCR Query expected static operand

I am trying to compare a value in my query with the returned values of a subquery.

The structure looks like the following:

SELECT * as node 

FROM [nt:unstructured]
WHERE node.[ATTR] in (SELECT otherNode.[ATTR] FROM … )

I get „expected: static operand“ error. Any suggestions on this?

PS: The query is a lot more complicated but the subquery returns the correct values. And also the main query returns the correct values (oc without the IN clausel)