diff --git a/include/jsoncons/source.hpp b/include/jsoncons/source.hpp
index bae4385c5b..3ce0519616 100644
--- a/include/jsoncons/source.hpp
+++ b/include/jsoncons/source.hpp
@@ -781,8 +781,12 @@ namespace jsoncons {
             return length - unread;
         }
     };
+#if __cplusplus >= 201703L
+// not needed for C++17
+#else
     template <class Source>
     constexpr std::size_t source_reader<Source>::max_buffer_length;
+#endif
 
     #if !defined(JSONCONS_NO_DEPRECATED)
     using bin_stream_source = binary_stream_source;