when moving a cursor, zss sometimes throws a javascript error

Steps to Reproduce

1. load the attached zul
2. select the 2nd sheet
3. move the cursor up and down around the last row

Current Result

>zss.wpd:15869 Uncaught TypeError: Cannot read property 'isMerged' of null
at init.doMouseMove_ (zss.wpd:15869)
at init.doMouseMove_ (zk.wpd:17694)
at _doEvt (zk.wpd:19556)
at HTMLDocument.<anonymous> (zk.wpd:19724)
at HTMLDocument.dispatch (zk.wpd:5157)
at HTMLDocument.elemData.handle (zk.wpd:4828)

Expected Result

no error