diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 5b7ff337e..2cc1fb20d 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -105,7 +105,7 @@ static inline Type& StructAfter(TObject &X)
   inline unsigned int get_size (void) const { return (size); }
 
 #define DEFINE_SIZE_UNION(size, _member) \
-  DEFINE_INSTANCE_ASSERTION (this->u._member.static_size == (size)); \
+  DEFINE_INSTANCE_ASSERTION (0*sizeof(this->u._member.static_size) + sizeof(this->u._member) == (size)); \
   static const unsigned int min_size = (size)
 
 #define DEFINE_SIZE_MIN(size) \