How to remove Chapter Members

I'm using the version of iMIS EMS 20.3.121.16, on one of the demoaisp instances.

Following the same logic for deleting a committee member, https://developer.imis.com/docs/working-with-committees. Removing MembershipDetails $values using a POST to /api/GroupMember endpoint doesn't appear to do anything and the status response is 201 created. Below is the JSON body I used.

{
"$type": "Asi.Soa.Membership.DataContracts.GroupMemberData, Asi.Contracts",
"MembershipDetails": {
"$type": "Asi.Soa.Membership.DataContracts.GroupMemberDetailDataCollection, Asi.Contracts",
"$values": [

                ]
            },
            "GroupMemberId": "CHAPT/HARRIS:19916",
            "Group": {
                "$type": "Asi.Soa.Membership.DataContracts.GroupSummaryData, Asi.Contracts",
                "GroupId": "CHAPT/HARRIS",
                "Name": "Harris County Chapter",
                "Description": "",
                "ParentIdentity": {
                    "$type": "Asi.Soa.Core.DataContracts.IdentityData, Asi.Contracts",
                    "EntityTypeName": "Public",
                    "IdentityElements": {
                        "$type": "System.Collections.ObjectModel.Collection`1[[System.String, mscorlib]], mscorlib",
                        "$values": [
                            "Public Groups"
                        ]
                    }
                },
                "GroupClass": {
                    "$type": "Asi.Soa.Membership.DataContracts.GroupClassSummaryData, Asi.Contracts",
                    "GroupClassId": "CHAPT",
                    "Name": "Chapter",
                    "Description": ""
                },
                "StatusCode": ""
            },
            "Party": {
                "$type": "Asi.Soa.Membership.DataContracts.PartySummaryData, Asi.Contracts",
                "CityName": "Wellington",
                "CountryName": "New Zealand",
                "CountrySubEntityName": "",
                "Email": "[email protected]",
                "Phone": "(021) 478-9883",
                "PartyId": "19916",
                "Id": "19916",
                "UniformId": "3d182cc3-73d9-4140-a2fb-8882a0091c13",
                "Status": {
                    "$type": "Asi.Soa.Membership.DataContracts.PartyStatusData, Asi.Contracts",
                    "PartyStatusId": "A",
                    "Name": "Active",
                    "Description": "Active"
                },
                "Name": "Mr. Aaron Stafford",
                "Sort": "STAFFORD, AARON",
                "IsMarkedForDelete": false
            },
            "IsActive": true
        }

Tried using PUT to /api/GroupMember/CHAPT%2FHARRIS%3A19916 and got back 500 Internal Error.