diff --git a/tests/data/plain_old_module.py b/tests/data/plain_old_module.py
index 49294464..608f27a5 100755
--- a/tests/data/plain_old_module.py
+++ b/tests/data/plain_old_module.py
@@ -12,7 +12,7 @@ class MyError(Exception):
 
 def get_sentinel_value():
     # Some proof we're even talking to the mitogen-test Docker image
-    return open('/etc/sentinel').read().decode()
+    return open('/etc/sentinel', 'rb').read().decode()
 
 
 def add(x, y):