Unable to change a definite employment contract to indefinite

Summary

I am unable to change a definite employment contract to indefinite

Content

Hi,

I am trying to add an indefinite employment contract to an employee who started with a definite contract (so with end-date). However I am required to add the extension period and contract end date.
Prior to 20A there was a workaround to make the end date and extension period not required. However if I try to do so I receive another error:

You must provide a positive integer value for Initial Duration and Extension Period (PER-1532430)

So it seems that the workaround is no longer supported. Can you please explain how to change the definite (temporary) contract to indefinete (permanent)? This is a critical process for all Dutch customers. Setting an end date to 31-12-4712 is not a solution.