diff --git a/vumi_twilio_api/tests/test_twilio_api.py b/vumi_twilio_api/tests/test_twilio_api.py
index 332debc..3592f43 100644
--- a/vumi_twilio_api/tests/test_twilio_api.py
+++ b/vumi_twilio_api/tests/test_twilio_api.py
@@ -60,6 +60,22 @@ def test_root_json(self):
         content = yield response.json()
         self.assertEqual(content, {})
 
+    @inlineCallbacks
+    def test_root_invalid_format(self):
+        response = yield self._server_request('.foo')
+        self.assertEqual(
+            response.headers.getRawHeaders('content-type'),
+            ['application/xml'])
+        self.assertEqual(response.code, 400)
+        content = yield response.content()
+        root = ET.fromstring(content)
+        [error_message, error_type] = sorted(root, key=lambda c: c.tag)
+        self.assertEqual(error_message.tag, 'error_message')
+        self.assertEqual(
+            error_message.text, "'foo' is not a valid request format")
+        self.assertEqual(error_type.tag, 'error_type')
+        self.assertEqual(error_type.text, 'UsageError')
+
 
 class TestServerFormatting(TestCase):
 
diff --git a/vumi_twilio_api/twilio_api.py b/vumi_twilio_api/twilio_api.py
index 0348db1..71df373 100644
--- a/vumi_twilio_api/twilio_api.py
+++ b/vumi_twilio_api/twilio_api.py
@@ -34,6 +34,13 @@ def teardown_application(self):
         yield self.webserver.loseConnection()
 
 
+class TwilioAPIUsageException(Exception):
+    """Called when in incorrect query is sent to the API"""
+    def __init__(self, message, format_='xml'):
+        super(TwilioAPIUsageException, self).__init__(message)
+        self.format_ = format_
+
+
 class TwilioAPIServer(object):
     app = Klein()
 
@@ -60,10 +67,23 @@ def format_json(dct):
     def _format_response(self, request, dct, format_):
         format_ = str(format_.lstrip('.').lower())
         func = getattr(
-            TwilioAPIServer, 'format_' + format_, TwilioAPIServer.format_xml)
+            TwilioAPIServer, 'format_' + format_, None)
+        if not func:
+            raise TwilioAPIUsageException(
+                '%r is not a valid request format' % format_)
         request.setHeader('Content-Type', 'application/%s' % format_)
         return func(dct)
 
+    @app.handle_errors(TwilioAPIUsageException)
+    def usage_exception(self, request, failure):
+        request.setResponseCode(400)
+        return self._format_response(
+            request, {
+                'error_type': 'UsageError',
+                'error_message': failure.value.message
+                },
+            failure.value.format_)
+
     @app.route('/', defaults={'format_': 'xml'}, methods=['GET'])
     @app.route('/<string:format_>', methods=['GET'])
     def root(self, request, format_):