--- a/layout/tools/reftest/output.py
+++ b/layout/tools/reftest/output.py
@@ -104,16 +104,17 @@ class OutputHandler(object):
     def __init__(self, log, utilityPath, symbolsPath=None):
         self.stack_fixer_function = get_stack_fixer_function(utilityPath, symbolsPath)
         self.log = log
 
     def __call__(self, line):
         # need to return processed messages to appease remoteautomation.py
         if not line.strip():
             return []
+        line = line.decode('utf-8', errors='replace')
 
         try:
             data = json.loads(line)
         except ValueError:
             self.verbatim(line)
             return [line]
 
         if isinstance(data, dict) and 'action' in data: