Error while trying to send a multipart request

Hello All,

I have been trying to send a multipart request using soapUI, but see the following error:
System.NullReferenceException: Object reference not set to an instance of an object. at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__1.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Web.Http.ApiController.<InvokeActionWithExceptionFilters>d__1.MoveNext().

I chose MediaType: multipart/form-data and added an attachment.
Here is the raw request:
POST http://hostname/ImportService/Imports/Upload/Users HTTP/1.1
Accept-Encoding: gzip,deflate
Content-Type: multipart/form-data; boundary="----=_Part_35_1122409331.1382987797294"
MIME-Version: 1.0
Content-Length: 47819
Host: hostname
Connection: Keep-Alive
User-Agent: Apache-HttpClient/4.1.1 (java 1.5)


------=_Part_35_1122409331.1382987797294
Content-Type: multipart/form-data
Content-Transfer-Encoding: 8bit

Content-Disposition: form-data; name="Request"

{
"CustomerGuid":"00000000-0000-0000-0000-000000000",
"ColumnMappings":[
{"FileColumnName":"Name","FileColumnNumber":0,"DtoPropertyName":"Name"},
{"FileColumnName":"Domain","FileColumnNumber":1,"DtoPropertyName":"Domain"},
{"FileColumnName":"FullName","FileColumnNumber":2,"DtoPropertyName":"FullName"},
{"FileColumnName":"Email","FileColumnNumber":3,"DtoPropertyName":"Email"},
{"FileColumnName":"Position","FileColumnNumber":4,"DtoPropertyName":"Position"},
{"FileColumnName":"PhoneNumber","FileColumnNumber":5,"DtoPropertyName":"PhoneNumber"},
{"FileColumnName":"Manager","FileColumnNumber":6,"DtoPropertyName":"Manager"},
{"FileColumnName":"Department","FileColumnNumber":7,"DtoPropertyName":"Department"},
{"FileColumnName":"Region","FileColumnNumber":8,"DtoPropertyName":"Region"},
{"FileColumnName":"Site","FileColumnNumber":9,"DtoPropertyName":"Site"},
{"FileColumnName":"Building","FileColumnNumber":10,"DtoPropertyName":"Building"},
{"FileColumnName":"Floor","FileColumnNumber":11,"DtoPropertyName":"Floor"},
{"FileColumnName":"Area","FileColumnNumber":12,"DtoPropertyName":"Area"}]
}
------=_Part_35_1122409331.1382987797294
Content-Type: application/octet-stream; name="PNZ&PUS 378 USERS.csv"
Content-Transfer-Encoding: binary
Content-Disposition: form-data; name="PNZ&PUS 378 USERS.csv"; filename="PNZ&PUS 378 USERS.csv"

Name,Domain,FullName,Email,Position,PhoneNumber,Manager,Department,Country,State,City,Building,Floor
"Pharosmaster","pnz","PharosMaster","Pharosmaster@pharos.co.nz","","","","Users - Utility","","","","",""
"krbtgt","pnz","krbtgt","","","","","Users - Utility","","","","",""
"Guest","pnz","Guest","","","","","Users - Utility","","","","",""

Thanks in advance