--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -286,17 +286,17 @@ class Warnings(MachCommandBase):
                     warning['column'], warning['flag'], warning['message']))
             else:
                 print('%s:%d [%s] %s' % (filename, warning['line'],
                     warning['flag'], warning['message']))
 
 @CommandProvider
 class GTestCommands(MachCommandBase):
     @Command('gtest', help='Run GTest unit tests.')
-    @CommandArgument('gtest_filter', default='*', nargs='?', metavar='gtest_filter',
+    @CommandArgument('gtest_filter', default=b"*", nargs='?', metavar='gtest_filter',
         help="test_filter is a ':'-separated list of wildcard patterns (called the positive patterns),"
              "optionally followed by a '-' and another ':'-separated pattern list (called the negative patterns).")
     @CommandArgument('--jobs', '-j', default='1', nargs='?', metavar='jobs', type=int,
         help='Run the tests in parallel using multiple processes.')
     @CommandArgument('--tbpl-parser', '-t', action='store_true',
         help='Output test results in a format that can be parsed by TBPL.')
     @CommandArgument('--shuffle', '-s', action='store_true',
         help='Randomize the execution order of tests.')