diff --git a/jlcchart/util.py b/jlcchart/util.py
index f644b14..2d3bde4 100644
--- a/jlcchart/util.py
+++ b/jlcchart/util.py
@@ -5,4 +5,4 @@ class ListConverter(BaseConverter):
         return value.split('+')
 
     def to_url(self, values):
-        return '+'.join(BaseConverter.to_url(value) for value in values)
+        return '+'.join(super(ListConverter, self).to_url(value) for value in values)