diff --git a/src/hb-ot-layout-gdef-private.hh b/src/hb-ot-layout-gdef-private.hh
index 456efca..85782b0 100644
--- a/src/hb-ot-layout-gdef-private.hh
+++ b/src/hb-ot-layout-gdef-private.hh
@@ -196,7 +196,7 @@
   CaretValueFormat3	format3[VAR];
   } u;
   public:
-  DEFINE_SIZE_MIN (2);
+  DEFINE_SIZE_UNION (2, format);
 };
 
 struct LigGlyph
@@ -312,7 +312,7 @@
   MarkGlyphSetsFormat1	format1[VAR];
   } u;
   public:
-  DEFINE_SIZE_MIN (2);
+  DEFINE_SIZE_UNION (2, format);
 };