diff --git a/lib/rails_admin/config/configurable.rb b/lib/rails_admin/config/configurable.rb
index b0faf69db..111599a2a 100644
--- a/lib/rails_admin/config/configurable.rb
+++ b/lib/rails_admin/config/configurable.rb
@@ -57,7 +57,8 @@ def register_instance_option(option_name, scope = self, &default)
 
           # If it's a boolean create an alias for it and remove question mark
           if option_name.end_with?('?')
-            scope.send(:define_method, "#{option_name.chop!}?") do
+            option_name = option_name.chop
+            scope.send(:define_method, "#{option_name}?") do
               send(option_name)
             end
           end