500 when using :iterate

[tomjscrypt]

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

[CaitlinV39]

AB#86957