diff --git a/lib/groupdate/magic.rb b/lib/groupdate/magic.rb
index e9c8148..d31431c 100644
--- a/lib/groupdate/magic.rb
+++ b/lib/groupdate/magic.rb
@@ -149,6 +149,7 @@ module Groupdate
       if missing_time_zone_support
         raise Groupdate::Error, "Be sure to install time zone support - https://github.com/ankane/groupdate#for-mysql"
       end
+      result = Hash[result.map { |k, v| [multiple_groups ? k[0...@group_index] + [cast_method.call(k[@group_index])] + k[(@group_index + 1)..-1] : cast_method.call(k), v] }]
 
       series(result, (options.key?(:default_value) ? options[:default_value] : 0), multiple_groups, reverse)
     end
--
libgit2 0.21.0