Function import '&1' cannot use return type '&2' if no data is returned

What causes this issue?

You are using a function import with a return type that cannot be used
for this function import if no data is returned.

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?

Function imports with cardinality 0..1 and an entity type as return type
have a restriction related to the key fields of the entity type in case
no data is returned. Read SAP note 2009874 for details.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.