Why does my modulo expression return no results?

CORRECTION:

I've got a big set of contour lines with 5 m and 10 m stepping from an ArcGIS File Geodatabase (wasn't a shape file, as previously stated). From time to time I need to modulo out the 5 m lines in the viewer. Given a column "contour" this should work (shouldn't it?):

"contour" % 10 = 0

However, it returns 0 lines with both, double or integer type. The column contains just values of xx0 and xx5.

Thanks for any hint!