diff --git a/testing/xpcshell/mach_commands.py b/testing/xpcshell/mach_commands.py
index 7646b1297b6e..ea1ea76e9f40 100644
--- a/testing/xpcshell/mach_commands.py
+++ b/testing/xpcshell/mach_commands.py
@@ -142,7 +142,7 @@ class XPCShellRunner(MozbuildObject):
             'debugger': debugger,
             'debuggerArgs': debuggerArgs,
             'debuggerInteractive': debuggerInteractive,
-            'on_message': (lambda obj, msg: xpcshell.log.info(msg)) \
+            'on_message': (lambda obj, msg: xpcshell.log.info(msg.decode('utf-8', 'replace'))) \
                             if test_path is not None else None,
         }