diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 383448c94a..aa6704d5c9 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -18,7 +18,7 @@ module ActiveRecord
 
       # Sets the primary key value
       def id=(value)
-        write_attribute(self.class.primary_key, value)
+        write_attribute(self.class.primary_key, value) if self.class.primary_key
       end
 
       # Queries the primary key value