diff --git a/tests/runtests.py b/tests/runtests.py
index 7fdc227018..2ab29262b2 100755
--- a/tests/runtests.py
+++ b/tests/runtests.py
@@ -20,8 +20,11 @@ from django.utils.deprecation import (
     RemovedInDjango19Warning, RemovedInDjango110Warning,
 )
 
+# Make deprecation warnings errors to ensure no usage of deprecated features.
 warnings.simplefilter("error", RemovedInDjango19Warning)
 warnings.simplefilter("error", RemovedInDjango110Warning)
+# Ignore known warnings in test dependencies.
+warnings.filterwarnings("ignore", "'U' mode is deprecated", DeprecationWarning, module='docutils.io')
 
 RUNTESTS_DIR = os.path.abspath(os.path.dirname(upath(__file__)))