Sorting by -lastUpdated results in "unknown sort parameter"

[plynchnlm]

I would like to retrieve the last 10 QuestionnaireResponses for a Patient, which I do by sorting by -lastUpdated (i.e., descending, with most recently modified first). This results in a 500 error. Interestingly, if I change "-lastUpdated" to "lastUpdated" (ascending order) it accepts the parameter name. I think there is a bug in handling the "-" prefix. Here is an example URL that shows the error:

http://test.fhir.org/r4/QuestionnaireResponse?subject=Patient%2Fihe-pcd&_include=QuestionnaireResponse%3Aquestionnaire&_sort=-_lastUpdated&_count=10