REST API Error - GET_REQUEST

I have successfully added requests using the API. Now I'm in the process of retrieving data from the REST interface.

I am accessing http://<service desk server>/sdpapi/request/<ticket number>
I am sending the following POST attributes:

OPERATION_NAME=GET_REQUEST
TECHNICIAN_KEY=<valid key here>

Here's the output I'm getting:

<operation name="GET_REQUEST">
      <result>
            <status>Failed</status>
            <message>Error when performing required operation.</message>
      </result>
</operation>

I'm not seeing any usable information as to what happened with the API. There are no log entries being returned in the system log.

Can anyone point me in a good direction?