diff --git a/Lib/fontTools/varLib/__init__.py b/Lib/fontTools/varLib/__init__.py
index 334e2623d..4379e666a 100644
--- a/Lib/fontTools/varLib/__init__.py
+++ b/Lib/fontTools/varLib/__init__.py
@@ -597,9 +597,15 @@ def _merge_OTL(font, model, master_fonts, axisTags):
 		GDEF = font['GDEF'].table
 		assert GDEF.Version <= 0x00010002
 	except KeyError:
-		font['GDEF']= newTable('GDEF')
+		font['GDEF'] = newTable('GDEF')
 		GDEFTable = font["GDEF"] = newTable('GDEF')
 		GDEF = GDEFTable.table = ot.GDEF()
+		GDEF.GlyphClassDef = None
+		GDEF.AttachList = None
+		GDEF.LigCaretList = None
+		GDEF.MarkAttachClassDef = None
+		GDEF.MarkGlyphSetsDef = None
+
 	GDEF.Version = 0x00010003
 	GDEF.VarStore = store