Internal: contract field is not filled

What causes this issue?

An internal error has occurred.

System Response

The process has been canceled.

How to fix this error?

Contact SAP with the following error message:
The field <LS>Contract</> is not filled.
It was determined in part 1 of module ISU_BILLING_PREPARE_UPDATE that
the contract field is not filled. A general error has occurred because
this field must always be filled.

Error message extract from SAP system. Copyright SAP SE.