diff --git a/nml/actions/action12.py b/nml/actions/action12.py
index d6d6214b..5d8c5e95 100644
--- a/nml/actions/action12.py
+++ b/nml/actions/action12.py
@@ -25,6 +25,7 @@ def write(self, file):
     'NORMAL' : 0,
     'SMALL'  : 1,
     'LARGE'  : 2,
+    'MONO'   : 3,
 }
 
 def parse_action12(font_glyphs):