diff --git a/include/wx/hashmap.h b/include/wx/hashmap.h
index 9b635e8a07..316abdec50 100644
--- a/include/wx/hashmap.h
+++ b/include/wx/hashmap.h
@@ -16,6 +16,8 @@
 #pragma interface "hashmap.h"
 #endif
 
+#include <stddef.h>         // for ptrdiff_t
+
 #include "wx/string.h"
 
 // private