diff --git a/lib/mongoid/fields/mappings.rb b/lib/mongoid/fields/mappings.rb
index 27100b8c5a..c9217c8d37 100644
--- a/lib/mongoid/fields/mappings.rb
+++ b/lib/mongoid/fields/mappings.rb
@@ -33,7 +33,7 @@ def for(klass, foreign_key = false)
           if match = klass.to_s.match(Regexp.new("^(#{ modules })?(\\w+)$"))
             "#{MODULE}::#{ match[2] }".constantize
           else
-            klass.to_s.constantize
+            klass
           end
         rescue NameError
           klass