LazySeq into #{}

t.clj

(def tat (atom #{}))

(class (products)) ; => clojure.lang.LazySeq

; works
(defn go []
  (let [ps (products)]
    (when-not (empty? ps)
      (reset! tat (into #{} ps))))

(go) ; resets tat correctly on every invocation

; crashes
(defn go []
  (let [ps (products)
        diff (set/difference ps @tat)]
    (when-not (empty? ps)
      (reset! tat (into #{} ps))))

(go) ; ClassCastException clojure.lang.LazySeq cannot be cast to clojure.lang.IPersistentSet  clojure.core/disj (core.clj:1454)

t2.clj

(def tat (atom #{}))

(class (products)) ; => clojure.lang.LazySeq

(defn go []
  (let [ps (products)
        diff (set/difference ps @tat)]
    (when-not (empty? ps)
      (reset! tat (into #{} ps))))

(go) ; works fine!
(go) ; ClassCastException clojure.lang.LazySeq cannot be cast to clojure.lang.IPersistentSet  clojure.core/disj (core.clj:1454)
(go) ; ClassCastException clojure.lang.LazySeq cannot be cast to clojure.lang.IPersistentSet  clojure.core/disj (core.clj:1454)

(def tat (atom #{}))

(go) ; works fine!
(go) ; ClassCastException clojure.lang.LazySeq cannot be cast to clojure.lang.IPersistentSet  clojure.core/disj (core.clj:1454)
; etc