diff --git a/Lib/python/pywstrings.swg b/Lib/python/pywstrings.swg
index 5c09fa4ed..8edcd0ac2 100644
--- a/Lib/python/pywstrings.swg
+++ b/Lib/python/pywstrings.swg
@@ -123,7 +123,7 @@ SWIG_AsWCharArray(PyObject *obj, wchar_t *val, size_t size)
   if (val) {
     PyErr_Format(PyExc_TypeError, 
 		 "a wchar_t array of maximum size %lud is expected", 
-		 size);
+		 (unsigned long) size);
   }
   return 0;
 }