diff --git a/graphviz/parameters/formats.py b/graphviz/parameters/formats.py
index c24d004de2..46ef1c6712 100644
--- a/graphviz/parameters/formats.py
+++ b/graphviz/parameters/formats.py
@@ -41,8 +41,7 @@
            'vrml',
            'wbmp',
            'webp',
-           'xlib',
-           'x11'}
+           'xlib', 'x11'}
 
 DEFAULT_FORMAT = 'pdf'