diff --git a/include/__debug b/include/__debug
index 2a4571024..f8c1129a3 100644
--- a/include/__debug
+++ b/include/__debug
@@ -17,6 +17,10 @@
 #pragma GCC system_header
 #endif
 
+#if defined(_LIBCPP_HAS_NO_NULLPTR)
+# include <cstddef>
+#endif
+
 #if _LIBCPP_DEBUG_LEVEL >= 1 || defined(_LIBCPP_BUILDING_LIBRARY)
 #   include <cstdlib>
 #   include <cstdio>