Error message function 'Value is empty and 'tolong' function cannot be applied at line Number 7'

Error message function 'Value is empty and 'tolong' function cannot be applied at line Number 7'

My workflow (see attachment) is triggered as soon as the expected weekly turnover is changed in the companies module to an amount higher than 0. After that, a function (see attachment) must ensure that this amount is copied to the registrations module. Only I get the following error:

Value is empty and 'tolong' function cannot be applied at line Number 7

When I run the function by clicking 'save and execute', it works. This same function also works fine when copying other fields between different modules. Do you have any idea what I can do about this?