Error on rejectChanges() call on ExtJs store

I have an ExtJs store that causes an error to raise when its rejectChanges() method is called. It looks like (from stepping through) that the rowIndex that the underlying ExtJs code is trying to hit is out of bounds. Why, I'm not sure. The closest search result I find took me to Commit/Reject changes in EditorGrid from the ExtJs forums, which was *not* helpful :(

I haven't worked out why it worked (as my code *does* already clear the filter earlier in the execution path), but, adding:

store.clearFilter(true);

Immediately before the call to rejectChanges() works a treat. Go figure!