diff --git a/include/rapidjson/document.h b/include/rapidjson/document.h
index b79abf7..7386773 100644
--- a/include/rapidjson/document.h
+++ b/include/rapidjson/document.h
@@ -1418,7 +1418,7 @@ public:
         \post IsString() == true && GetString() != s.data() && strcmp(GetString(),s.data() == 0 && GetStringLength() == s.size()
         \note Requires the definition of the preprocessor symbol \ref RAPIDJSON_HAS_STDSTRING.
     */
-    GenericValue& SetString(const std::basic_string<Ch>& s, Allocator& allocator) { return SetString(s.data(), s.size(), allocator); }
+    GenericValue& SetString(const std::basic_string<Ch>& s, Allocator& allocator) { return SetString(s.data(), SizeType(s.size()), allocator); }
 #endif
 
     //@}