diff --git a/include/tclap/StandardTraits.h b/include/tclap/StandardTraits.h
index 46d7f6f..117057b 100644
--- a/include/tclap/StandardTraits.h
+++ b/include/tclap/StandardTraits.h
@@ -123,8 +123,9 @@ struct ArgTraits<unsigned char> {
     typedef ValueLike ValueCategory;
 };
 
-// Microsoft implements size_t awkwardly. 
-#if defined(_MSC_VER) && defined(_M_X64)
+// Microsoft implements size_t awkwardly.
+// Studio 2005 introduces unsigned long long, which conflicts with the size_t template
+#if defined(_MSC_VER) && (_MSC_VER < 1400) && defined(_M_X64)
 /**
  * size_ts have value-like semantics.
  */