parametercannotbevalueoptional error from core_user_get_users() via xmlrpc

I have a service integration between an external system to moodle. The service utilizes the Charles Cook xml-rpc library to sync registration data between the registration system and moodle. After the Moodle instance was upgraded to version 2.7(Build: 20140522), moodle web services have started returning the following error when calling the core_user_get_user web service function:

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse><fault><value><struct><member><name>faultCode</name><value><int>17038686</int></value></member><member><name>faultString</name><value><string>error/parametercannotbevalueoptional | ERRORCODE: parametercannotbevalueoptional</string></value></member></struct></value></fault></methodResponse>

Our web service passes the StudentID to the method to determine if the student has already been created in moodle. StudentID services as the related field between the 2 systems. I have tried passing other fields to the moodle services to see if it was just this field that shows the issue, but I have been unable to get any response besides the error listed above.

Here is the fiddler output of the request:

<?xml version="1.0"?>

<methodCall>

  <methodName>core_user_get_users</methodName>

  <params>

    <param>

      <value>

        <array>

          <data>

            <value>

              <struct>

                <member>

                  <name>key</name>

                  <value>

                    <string>id</string>

                  </value>

                </member>

                <member>

                  <name>value</name>

                  <value>

                    <string>879</string>

                  </value>

                </member>

              </struct>

            </value>

          </data>

        </array>

      </value>

    </param>

  </params>

</methodCall>

Is this a known issue or has there been a change in the way this method should be called? I didn't see any documentation updates that describe this error or any possible workaround.

It should be noted that the external system has no knowledge of the internal moodle id of a student, so the studentID field is the most reliable means of relating profiles between the 2 systems.