insert a column into a merged cell causes a javascript error

Steps to Reproduce

1. load the attached file
2. insert a column at D

Current Result

zssex.wpd:4426 Uncaught TypeError: Cannot read properties of undefined (reading 'custColWidth')
at Array.<anonymous> (zssex.wpd:4426:53)
at zss.wpd:42294:13

Expected Result

no js error

Debug Information

• should be self.sheet