diff --git a/src/calibre/debug.py b/src/calibre/debug.py
index fe0fe9e6de..15b00a7d61 100644
--- a/src/calibre/debug.py
+++ b/src/calibre/debug.py
@@ -79,9 +79,9 @@ Everything after the -- is passed to the script.
         'Run a plugin that provides a command line interface. For example:\n'
         'calibre-debug -r "Add Books" -- file1 --option1\n'
         'Everything after the -- will be passed to the plugin as arguments.'))
-    parser.add_option('--diff', help=_(
+    parser.add_option('--diff', action='store_true', default=False, help=_(
         'Run the calibre diff tool. For example:\n'
-        'calibre-debug --diff -- file1 file2'))
+        'calibre-debug --diff file1 file2'))
 
     return parser