diff --git a/src/io/json.cpp b/src/io/json.cpp
index b9afc75..666af76 100644
--- a/src/io/json.cpp
+++ b/src/io/json.cpp
@@ -631,7 +631,7 @@ namespace fc
               to_stream(os, v.get_object(), format, max_depth - 1 );
               return;
          default:
-            FC_THROW_EXCEPTION( fc::invalid_arg_exception, "Unsupported variant type: " + v.get_type() );
+            FC_THROW_EXCEPTION( fc::invalid_arg_exception, "Unsupported variant type: " + std::to_string(v.get_type()) );
       }
    }