diff --git a/lib/mongoid/fields/foreign_key.rb b/lib/mongoid/fields/foreign_key.rb
index 2b8b558a90..815d5cdc99 100644
--- a/lib/mongoid/fields/foreign_key.rb
+++ b/lib/mongoid/fields/foreign_key.rb
@@ -134,6 +134,8 @@ def resizable?
       # @return [ Object ] The converted object.
       def mongoize_foreign_key(object)
         if type == Array || type == Set
+          object = object.to_a if type == Set || object.is_a?(Set)
+
           if object.resizable?
             object.blank? ? object : association.convert_to_foreign_key(object)
           else