Prevent possible division by zero

This might cause problems on certain platforms. While hard
to reproduce it seems safer to add the check.

Note that we do not get the alternating pattern if there is nothing
set on the model, but this was already the case.

Change-Id: I984df9cfef30b4de0e72e8ccf14cc65bddbf04ef
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>