Problem with retrieving UNSRootB object

When I invoked code:

If Session.Source.TryGet( Query.From("UNSRootB") _ 
                               .Where(f.UidComparison("UID_UNSRootB", UID_UNSRootB, CompareOperator.Equal)) _
                               .SelectNonLobs, UNSRootB )
    ...
End If

TryGet results with False (object could not be found).

On the other hand when I execute SQL query:

SELECT * FROM UNSRootB WHERE UID_UNSRootB = '<UID_UNSRootB>'

it returns one row.

IDM ver. 7.1.2