@stale_state should be nil when a model isn't saved.

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -77,7 +77,7 @@ module ActiveRecord
         end
 
         def stale_state
-          owner[reflection.foreign_key].to_s
+          owner[reflection.foreign_key] && owner[reflection.foreign_key].to_s
         end
     end
   end

activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb

@@ -27,7 +27,8 @@ module ActiveRecord
         end
 
         def stale_state
-          [super, owner[reflection.foreign_type].to_s]
+          foreign_key = super
+          foreign_key && [foreign_key.to_s, owner[reflection.foreign_type].to_s]
         end
     end
   end

activerecord/lib/active_record/associations/through_association.rb

@@ -62,7 +62,7 @@ module ActiveRecord
         # properly support stale-checking for nested associations.
         def stale_state
           if through_reflection.macro == :belongs_to
-            owner[through_reflection.foreign_key].to_s
+            owner[through_reflection.foreign_key] && owner[through_reflection.foreign_key].to_s
           end
         end