Include Test Custom Field in executeSearch

We're retrieving test executions using "executeSearch", example below...

https://server/rest/zapi/latest/zql/executeSearch?zqlQuery=project='Project' AND fixVersion='Release' AND executionStatus IN ('PASS','FAIL')

We noticed that the result of the above command includes test fields (i.e. description, priority, etc). Now our Tests have some custom fields, is there a way to include custom fields to the results of the command above? We tried the command below but its returning empty customfields.

https://server/rest/zapi/latest/zql/executeSearch?zqlQuery=project='Project' AND fixVersion='Release' AND executionStatus IN ('PASS','FAIL')&expand=customFields

Thanks!