diff --git a/src/hb-null.hh b/src/hb-null.hh
index 725db5024..674b4dffc 100644
--- a/src/hb-null.hh
+++ b/src/hb-null.hh
@@ -46,13 +46,13 @@
  * https://stackoverflow.com/questions/7776448/sfinae-tried-with-bool-gives-compiler-error-template-argument-tvalue-invol
  */
 
-template <typename T, typename B>
+template <typename T, typename>
 struct _hb_null_size : hb_integral_constant<unsigned, sizeof (T)> {};
 template <typename T>
-struct _hb_null_size<T, hb_bool_constant<true || sizeof (T::min_size)>> : hb_integral_constant<unsigned, T::null_size> {};
+struct _hb_null_size<T, hb_void_t<decltype (T::min_size)>> : hb_integral_constant<unsigned, T::null_size> {};
 
 template <typename T>
-using hb_null_size = _hb_null_size<T, hb_true_type>;
+using hb_null_size = _hb_null_size<T, void>;
 #define hb_null_size(T) hb_null_size<T>::value
 
 /* These doesn't belong here, but since is copy/paste from above, put it here. */
@@ -60,12 +60,12 @@ using hb_null_size = _hb_null_size<T, hb_true_type>;
 /* hb_static_size (T)
  * Returns T::static_size if T::min_size is defined, or sizeof (T) otherwise. */
 
-template <typename T, typename B>
+template <typename T, typename>
 struct _hb_static_size : hb_integral_constant<unsigned, sizeof (T)> {};
 template <typename T>
-struct _hb_static_size<T, hb_bool_constant<true || sizeof (T::min_size)>> : hb_integral_constant<unsigned, T::static_size> {};
+struct _hb_static_size<T, hb_void_t<decltype (T::min_size)>> : hb_integral_constant<unsigned, T::static_size> {};
 template <typename T>
-using hb_static_size = _hb_static_size<T, hb_true_type>;
+using hb_static_size = _hb_static_size<T, void>;
 #define hb_static_size(T) hb_static_size<T>::value