I haven't been able to duplicate this behaviour.

Try creating a new specification or opening one of the spec Templates from the File / Open Template menu, then the same process that is giving you the error. If this works, then the problem lies with your invoice specification(s).

If this also gives the same 'division by zero' error, then you may have had an installation problem where the designer program is out of synch with the rest. What is the file date and version of your OEWDESGN.EXE program in the \softrak\oewin directory?