empty-counted-sorted-set-disj-get.patch

diff --git a/src/main/clojure/clojure/data/finger_tree.clj b/src/main/clojure/clojure/data/finger_tree.clj
index bc00343..557aaaf 100644
--- a/src/main/clojure/clojure/data/finger_tree.clj
+++ b/src/main/clojure/clojure/data/finger_tree.clj
@@ -537,13 +537,17 @@
     (count [_] (:len (measured tree)))
   IPersistentSet
     (disjoin [this k]
-      (let [[l x r] (split-tree tree #(>= 0 (cmpr k (:right %))))]
-        (if (= x k)
-          (CountedSortedSet. cmpr (ft-concat l r))
-          this)))
+      (if (empty? tree)
+        this
+        (let [[l x r] (split-tree tree #(>= 0 (cmpr k (:right %))))]
+          (if (= x k)
+            (CountedSortedSet. cmpr (ft-concat l r))
+            this))))
     (get [_ k]
-      (let [x (second (split-tree tree #(>= 0 (cmpr k (:right %)))))]
-        (when (= x k) k)))
+      (if (empty? tree)
+        nil
+        (let [x (second (split-tree tree #(>= 0 (cmpr k (:right %)))))]
+          (when (= x k) k))))
   Indexed
     (nth [this n notfound] (if (< -1 n (:len (measured tree)))
                              (second (split-tree tree #(> (:len %) n)))