ASP.NET: Non-Json Response in API Action

Hi.
I have an API which takes “text” as content type and returns a plain text string as response similar to: OK:123123:6786978

I am getting following error:
"ClassName": "Newtonsoft.Json.JsonReaderException", "Message": "Unexpected character encountered while parsing value: O. Path '', line 0, position 0.",

It seems API action is unable to process a response which is not a JSON. Is that so? Or is there any way I can get this response?