--- a/taskcluster/mach_commands.py
+++ b/taskcluster/mach_commands.py
@@ -38,17 +38,17 @@ class ShowTaskGraphSubCommand(SubCommand
             CommandArgument('--verbose', '-v', action="store_true",
                             help="include debug-level logging output"),
             CommandArgument('--json', '-J', action="store_const",
                             dest="format", const="json",
                             help="Output task graph as a JSON object"),
             CommandArgument('--labels', '-L', action="store_const",
                             dest="format", const="labels",
                             help="Output the label for each task in the task graph (default)"),
-            CommandArgument('--parameters', '-p', default=False,
+            CommandArgument('--parameters', '-p', default=None,
                             help="parameters file (.yml or .json; see "
                                  "`taskcluster/docs/parameters.rst`)`"),
             CommandArgument('--no-optimize', dest="optimize", action="store_false",
                             default="true",
                             help="do not remove tasks from the graph that are found in the "
                             "index (a.k.a. optimize the graph)"),
             CommandArgument('--tasks-regex', '--tasks', default=None,
                             help="only return tasks with labels matching this regular "