diff --git a/include/wx/hashmap.h b/include/wx/hashmap.h
index e774c0a193..5bd69cea77 100644
--- a/include/wx/hashmap.h
+++ b/include/wx/hashmap.h
@@ -18,6 +18,8 @@
 
 #include "wx/string.h"
 
+#include <stddef.h>             // for ptrdiff_t
+
 // private
 struct WXDLLEXPORT _wxHashTable_NodeBase
 {