500 when using :iterate

Following query causes 500:

{{fhirEndpoint}}Observation/?_revinclude=Observation:part-of&_include=Observation:focus&_include:iterate=Observation:focus

using same query but on performer instead of focus is working:
{{fhirEndpoint}}Observation/?_revinclude=Observation:part-of&_include=Observation:focus&_include:iterate=Observation:performer

What is interesting one can observe this on empty server without any Observation.

Also this gives 500 (after removing ''&_include=Observation:focus" from first query):

{{fhirEndpoint}}Observation/?_revinclude=Observation:part-of&_include:iterate=Observation:focus

Reproduce the listed FHIR queries against an empty server, comparing :focus with :performer and testing the cases with and without _include=Observation:focus. Trace the API handling for _include:iterate and verify that these requests no longer return HTTP 500; the issue names no files or tests.

Written by the indexing model from the issue text.