diff --git a/lib/ransack/nodes/grouping.rb b/lib/ransack/nodes/grouping.rb
index 15c1c95a7..38d794df0 100644
--- a/lib/ransack/nodes/grouping.rb
+++ b/lib/ransack/nodes/grouping.rb
@@ -108,7 +108,7 @@ def groupings=(groupings)
       alias :g= :groupings=
 
       def method_missing(method_id, *args)
-        method_name = method_id.to_s
+        method_name = method_id.to_s.dup
         writer = method_name.sub!(/\=$/, ''.freeze)
         if attribute_method?(method_name)
           if writer