diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 1e97bac0189..8ec3d719dc9 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -1081,7 +1081,8 @@ class POSIXProcessTestCase(BaseTestCase):
                              stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         stdout, stderr = p.communicate()
         self.assertEqual(0, p.returncode, "sigchild_ignore.py exited"
-                         " non-zero with this error:\n%s" % stderr)
+                         " non-zero with this error:\n%s" %
+                         stderr.decode('utf8'))
 
 
 @unittest.skipUnless(mswindows, "Windows specific tests")