diff --git a/include/mapnik/json/unicode_string_grammar_x3_def.hpp b/include/mapnik/json/unicode_string_grammar_x3_def.hpp
index 00a5932b8..c1f6a39c0 100644
--- a/include/mapnik/json/unicode_string_grammar_x3_def.hpp
+++ b/include/mapnik/json/unicode_string_grammar_x3_def.hpp
@@ -83,8 +83,8 @@ using x3::hex;
 using x3::eol;
 using x3::no_skip;
 
-x3::uint_parser<uchar, 16, 4, 4> hex4;
-x3::uint_parser<uchar, 16, 8, 8> hex8;
+x3::uint_parser<uchar, 16, 4, 4> const hex4;
+x3::uint_parser<uchar, 16, 8, 8> const hex8;
 
 // start rule
 unicode_string_grammar_type const unicode_string("Unicode String");