diff --git a/include/tclap/StandardTraits.h b/include/tclap/StandardTraits.h
index f63ed65..46d7f6f 100644
--- a/include/tclap/StandardTraits.h
+++ b/include/tclap/StandardTraits.h
@@ -123,6 +123,18 @@ struct ArgTraits<unsigned char> {
     typedef ValueLike ValueCategory;
 };
 
+// Microsoft implements size_t awkwardly. 
+#if defined(_MSC_VER) && defined(_M_X64)
+/**
+ * size_ts have value-like semantics.
+ */
+template<>
+struct ArgTraits<size_t> {
+    typedef ValueLike ValueCategory;
+};
+#endif
+
+
 #ifdef HAVE_LONG_LONG
 /**
  * unsigned long longs have value-like semantics.