diff --git a/lib/simple_form/components/readonly.rb b/lib/simple_form/components/readonly.rb
index 504072e1..f4a986c2 100644
--- a/lib/simple_form/components/readonly.rb
+++ b/lib/simple_form/components/readonly.rb
@@ -15,7 +15,7 @@ module SimpleForm
       def readonly_attribute?
         object.class.respond_to?(:readonly_attributes) &&
           object.persisted? &&
-          object.class.readonly_attributes.include?(attribute_name)
+          object.class.readonly_attributes.include?(attribute_name.to_s)
       end
     end
   end
diff --git a/test/support/models.rb b/test/support/models.rb
index bb5727fe..6e1bd9d6 100644
--- a/test/support/models.rb
+++ b/test/support/models.rb
@@ -130,7 +130,7 @@ class User
   end
 
   def self.readonly_attributes
-    [:credit_card]
+    ["credit_card"]
   end
 end