Field modification - Working hours throws error

Summary:

For the business object Worker Assignment, I put in place a field modification for the working hours which throws an error.

Content (required):

We'd like the working hours to be defaulted by the current working hours from the existing assignment instead of having them overwritten by the standard working hours. It works, however, there is an error message when trying to proceed. Please find attached the rules & the error message. Anyone an idea how this can be solved? Many thanks in advance!

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