diff --git a/Lib/fontTools/ttLib/tables/T_S_I__5.py b/Lib/fontTools/ttLib/tables/T_S_I__5.py
index cf29700a9..2e2305a98 100644
--- a/Lib/fontTools/ttLib/tables/T_S_I__5.py
+++ b/Lib/fontTools/ttLib/tables/T_S_I__5.py
@@ -23,7 +23,7 @@ class table_T_S_I__5(DefaultTable.DefaultTable):
 		glyphNames = ttFont.getGlyphOrder()
 		a = array.array("H")
 		for i in range(len(glyphNames)):
-			a.append(self.glyphGrouping[glyphNames[i]])
+			a.append(self.glyphGrouping.get(glyphNames[i], 0))
 		if sys.byteorder != "big":
 			a.byteswap()
 		return a.tostring()