Returning a text file from ServiceStack

request.http

GET /DocData/api/replication/db.json?DbName=test HTTP/1.1
Host: localhost
Connection: keep-alive
Cache-Control: max-age=0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36
Accept-Encoding: gzip, deflate, sdch
Accept-Language: en-US,en;q=0.8
Cookie: ss-id=hO2XaZXMotKtMOEOE4WF; ss-pid=9MwQlX6VDITl5UcGsLfH; DocStudySetId=7945916; AuthSession=

response.http

HTTP/1.1 500 NullReferenceException
Cache-Control: private
Content-Type: application/json; charset=utf-8
Vary: Accept
Server: Microsoft-IIS/7.5
X-Powered-By: ServiceStack/4.035 Win32NT/.NET
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization
X-AspNet-Version: 4.0.30319
Set-Cookie: AuthSession=; path=/; HttpOnly
X-Powered-By: ASP.NET
Date: Thu, 15 Jan 2015 16:35:09 GMT
Content-Length: 1330

response.json

{
   "ResponseStatus":{
      "ErrorCode":"NullReferenceException",
      "Message":"Object reference not set to an instance of an object.",
      "StackTrace":"[DocDtoReplication: 1/15/2015 2:44:03 PM]:\n[REQUEST: {DbName:statins}]\nSystem.NullReferenceException: Object reference not set to an instance of an object.\r\n   at ServiceStack.HttpResult.get_IsPartialRequest()\r\n   at lambda_method(Closure , HttpResult )\r\n   at ServiceStack.Text.Common.WriteType`2.WriteProperties(TextWriter writer, Object value)\r\n   at ServiceStack.Text.JsonSerializer.SerializeToString(Object value, Type type)\r\n   at ServiceStack.Text.JsonSerializer.SerializeToString[T](T value)\r\n   at ServiceStack.Serialization.JsonDataContractSerializer.SerializeToString[T](T obj)\r\n   at ServiceStack.Host.ContentTypes.SerializeToString(IRequest req, Object response)\r\n   at ServiceStack.RequestExtensions.ToOptimizedResult[T](IRequest request, T dto)\r\n   at DocDataWebApp.Dre.Config.DocServiceRunner`1.OnAfterExecute(IRequest requestContext, Object response) in c:\\doc\\DocDataWebApp\\Dre\\Config\\DocServiceRunner.cs:line 26\r\n   at ServiceStack.Host.ServiceRunner`1.AfterEachRequest(IRequest requestContext, TRequest request, Object response)\r\n   at ServiceStack.Host.ServiceRunner`1.Execute(IRequest request, Object instance, TRequest requestDto)",
      "Errors":[

      ]
   }
}

server.cs

            var fileName = String.Concat( @"~/backups/", request.DbName, ".txt" );
            var fileInfo = new FileInfo( fileName.MapHostAbsolutePath() );

            return new HttpResult( fileInfo, asAttachment: false );