diff --git a/include/mapnik/json/generic_json_grammar_x3.hpp b/include/mapnik/json/generic_json_grammar_x3.hpp
index 27cd92c44..2da057596 100644
--- a/include/mapnik/json/generic_json_grammar_x3.hpp
+++ b/include/mapnik/json/generic_json_grammar_x3.hpp
@@ -43,17 +43,17 @@ BOOST_SPIRIT_DECLARE(generic_json_grammar_type);
 BOOST_SPIRIT_DECLARE(generic_json_key_value_type);
 }
 
-auto assign = [](auto const& ctx)
+inline auto assign = [](auto const& ctx)
 {
     _val(ctx) = _attr(ctx);
 };
 
-auto assign_key = [](auto const& ctx)
+inline auto assign_key = [](auto const& ctx)
 {
-    std::get<0>(_val(ctx)) = _attr(ctx);
+    std::get<0>(_val(ctx)) = std::move(_attr(ctx));
 };
 
-auto assign_value = [](auto const& ctx)
+inline auto assign_value = [](auto const& ctx)
 {
     std::get<1>(_val(ctx)) = std::move(_attr(ctx));
 };