Floating-rate bonds have very low effective duration; not modified duration, and not Macaulay duration.
Recall from Level I that when a bond’s coupon rate is the same as its YTM, it sells at par. Because a floating-rate bond’s coupon rate resets to market interest rates, its price is always extremely close to par. (It may vary a little between coupon payment dates, but not much.) Very little price change means very short effective duration.