diff --git a/.travis.yml b/.travis.yml
index f87320c..26e0c65 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: python
 python:
  - "2.7"
- - "3.3"
  - "3.4"
  - "3.5"
 env:
diff --git a/pyramid_urireferencer/views.py b/pyramid_urireferencer/views.py
index c2cfe0f..b7f5670 100644
--- a/pyramid_urireferencer/views.py
+++ b/pyramid_urireferencer/views.py
@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-
+from pyramid.httpexceptions import HTTPBadRequest
 from pyramid.view import view_config
 from pyramid_urireferencer import get_referencer
 
@@ -20,4 +20,6 @@ class RestView(ApplicatieView):
 class ReferencesPluginView(RestView):
     @view_config(route_name='references', renderer='json_item', accept='application/json')
     def get_references(self):
+        if not self.request.params.get('uri'):
+            raise HTTPBadRequest('Uri is required.')
         return get_referencer(self.request.registry).references(self.request.params.get('uri'), self.request)
diff --git a/requirements-dev.txt b/requirements-dev.txt
index a563594..fce690d 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -2,6 +2,7 @@
 --requirement requirements.txt
 
 #testing
+attrs==19.1.0
 pytest==3.4.2
 pytest-cov==2.5.1
 webtest==2.0.29
diff --git a/tests/test_views.py b/tests/test_views.py
index 33a0005..0025ef8 100644
--- a/tests/test_views.py
+++ b/tests/test_views.py
@@ -2,11 +2,22 @@
 
 import json
 import unittest
+
+try:
+    from unittest.mock import Mock, patch
+except ImportError:
+    from mock import Mock, patch
+
 import httpretty
 from pyramid import testing
-from pyramid_urireferencer import IReferencer, Referencer, _add_referencer, get_referencer
-from pyramid_urireferencer.views import ReferencesPluginView
+from pyramid.httpexceptions import HTTPBadRequest
+
+from pyramid_urireferencer import IReferencer
+from pyramid_urireferencer import Referencer
+from pyramid_urireferencer import _add_referencer
+from pyramid_urireferencer import get_referencer
 from pyramid_urireferencer.models import RegistryResponse
+from pyramid_urireferencer.views import ReferencesPluginView
 
 try:
     from urllib import urlencode
@@ -64,6 +75,18 @@ def test_is_referenced(self):
         httpretty.disable()  # disable afterwards, so that you will have no problems in code that uses that socket module
         httpretty.reset()
 
+    def test_no_uri(self):
+        request = Mock(params={'uri': ''})
+        view = ReferencesPluginView(request)
+        with self.assertRaises(HTTPBadRequest):
+            view.get_references()
+
+    def test_uri_none(self):
+        request = Mock(params={'uri': None})
+        view = ReferencesPluginView(request)
+        with self.assertRaises(HTTPBadRequest):
+            view.get_references()
+
 
 class TestReferencer(Referencer):
     def references(self, uri, request):
@@ -71,5 +94,3 @@ def references(self, uri, request):
 
     def get_uri(self, request):
         return 'https://id.erfgoed.net/resources/1'
-
-