Return if not expires at

@@ -203,6 +203,7 @@ module ActsAsChargeable
     end
 
     def var_expired_by(time)
+      return false unless var_payment_expires_at
       return var_payment_expires_at + time < Time.zone.now && !var_payed?
     end
 
@@ -222,4 +223,4 @@ end
 
 if defined? ActiveRecord::Base
   ActiveRecord::Base.send(:include, ActsAsChargeable)
-end
+end
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