--- a/python/mach_commands.py
+++ b/python/mach_commands.py
@@ -203,18 +203,18 @@ class MachCommands(MachCommandBase):
         def _line_handler(line):
             if not file_displayed_test:
                 output = ('Ran' in line or 'collected' in line or
                           line.startswith('TEST-'))
                 if output:
                     file_displayed_test.append(True)
 
             # Hack to make sure treeherder highlights pytest failures
-            if 'FAILED' in line.rsplit(' ', 1)[-1]:
-                line = line.replace('FAILED', 'TEST-UNEXPECTED-FAIL')
+            if b'FAILED' in line.rsplit(b' ', 1)[-1]:
+                line = line.replace(b'FAILED', b'TEST-UNEXPECTED-FAIL')
 
             _log(line)
 
         _log(test_path)
         cmd = [self.virtualenv_manager.python_path, test_path]
         env = os.environ.copy()
         env[b'PYTHONDONTWRITEBYTECODE'] = b'1'