diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index b9880bad719b..b1b7a2c610f1 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -844,7 +844,7 @@ def findTestMediaDevices(log):
         log.error('Could not list currently loaded modules')
         return None
 
-    null_sink = [x for x in o.splitlines() if 'module-null-sink' in x]
+    null_sink = [x for x in o.splitlines() if b'module-null-sink' in x]
 
     if not null_sink:
         try: