RAP Contract Violation for FAILED: READ BY ASSOCIATION with &1

What causes this issue?

READ BY ASSOCIATION Contract:
Entries in MAPPED, FAILED and RESULT contain always a sub-set of the
input keys. Input keys which are not part of RESULT must be part of
FAILED.
The contract of the following read by association operation has been
violated for entity &V2&:
A READ BY ASSOCIATION with a non-existing or empty key must return a
failed entry with reason "not found".
Info:
Corresponding entries in reported can be returned, but this is not
mandatory. Additional entries can be reported, referring to keys outside
of the input keys.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Please check the response of your behavior implementation and adopt it
according to the contract.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.