diff --git a/src/hb-aat-layout-morx-table.hh b/src/hb-aat-layout-morx-table.hh
index 5dec189fd..f13b55cab 100644
--- a/src/hb-aat-layout-morx-table.hh
+++ b/src/hb-aat-layout-morx-table.hh
@@ -195,6 +195,8 @@ struct ContextualSubtable
 				 * marked glyph (use 0xFFFF for none). */
     HBUINT16	currentIndex;	/* Index of the substitution table for the
 				 * current glyph (use 0xFFFF for none). */
+    public:
+    DEFINE_SIZE_MIN (4);
   };
 
   inline bool apply (hb_apply_context_t *c) const