Using non-sequential groups throws a fatal TypeError

This is probably another edge-case, but if you pass a non-sequential array to data.groups, a TypeError is thrown. See [jsFiddle](http://jsfiddle.net/aendrew/b3dh230j/1/).

Start by reproducing the TypeError with the linked jsFiddle and a non-sequential array passed to data.groups. Trace the data.groups handling to identify the failing path, then verify that the same input no longer throws and that existing group behavior remains intact.

Written by the indexing model from the issue text.