diff --git a/c++/src/capnp/message.h b/c++/src/capnp/message.h
index 983761dfb3..fdce80c33d 100644
--- a/c++/src/capnp/message.h
+++ b/c++/src/capnp/message.h
@@ -128,9 +128,9 @@ class MessageReader {
   ReaderOptions options;
 
 #if defined(__EMSCRIPTEN__) || (defined(__APPLE__) && (defined(__ppc__) || defined(__i386__)))
-  static constexpr size_t arenaSpacePadding = 19;
+  static constexpr size_t arenaSpacePadding = 20;
 #else
-  static constexpr size_t arenaSpacePadding = 18;
+  static constexpr size_t arenaSpacePadding = 19;
 #endif
 
   // Space in which we can construct a ReaderArena.  We don't use ReaderArena directly here