SetBusinessSystemUser Web API call failure

Hi all, 

I'm testing some actions using the WebAPI using both Postman and a test app. I'm seeing two different results. The Postman call to the web api works. The app's PostAsync fails with an error of "An error occurred while validating input parameters: Microsoft.OData.ODataException: The parameter 'ReassignPrinciple' in the request payload is not a valid parameter for the operation 'SetBusinessSystemUser'.

I've verified the values are the same for both calls. I'm obviously missing something here and just not seeing the difference.

Postman call

test app code: 

JObject businessUnit = JObject.Parse(@"{businessunitid: '" + businessUnitId + @"', '@odata.type': 'Microsoft.Dynamics.CRM.businessunit' }");
JObject reassignPrinciple = JObject.Parse(@"{systemuserid: '" + systemUserId + @"', '@odata.type': 'Microsoft.Dynamics.CRM.systemuser' }");

JObject bodyJson = new JObject()
{
{"BusinessUnit", businessUnit },
{"ReassignPrinciple", reassignPrinciple }
};

var stringContent = new StringContent(bodyJson.ToString(), System.Text.Encoding.UTF8, "application/json");
stringContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json");

var response = await _httpClient.PostAsync($"systemusers({systemUserId})/Microsoft.Dynamics.CRM.SetBusinessSystemUser", stringContent);

test app body string:

"{\r\n \"BusinessUnit\": {\r\n \"businessunitid\": \"a450c60b-345c-eb11-a812-000d3a8a8e8c\",\r\n \"@odata.type\": \"Microsoft.Dynamics.CRM.businessunit\"\r\n },\r\n \"ReassignPrinciple\": {\r\n \"systemuserid\": \"51ca36a1-600e-eb11-a814-000d3a8dfcac\",\r\n \"@odata.type\": \"Microsoft.Dynamics.CRM.systemuser\"\r\n }\r\n}"

Any help is appreciated. Thanks in advance.