diff --git a/src/net/cgrand/xforms.clj b/src/net/cgrand/xforms.clj
index 44bb5fc..fa87130 100644
--- a/src/net/cgrand/xforms.clj
+++ b/src/net/cgrand/xforms.clj
@@ -142,39 +142,44 @@
    Partitions contain the \"value part\" (as returned by vfn) of each item.
    The resulting transformed items are wrapped back into a \"pair\" using the pair function.
    Default values for kfn, vfn and pair are first, second (or identity if kfn is specified) and vector."
-  ([xform] (by-key key' val' vector xform))
+  ([xform] (by-key nil nil vector xform))
   ([kfn xform] (by-key kfn identity vector xform))
   ([kfn vfn xform] (by-key kfn vfn vector xform))
   ([kfn vfn pair xform]
-    (fn [rf]
-      (let [make-rf (if pair
-                      (if-some [rf (when (identical? vector pair) (some-kvrf rf))]
-                        (fn [k] (fn ([acc] acc) ([acc v] (rf acc k v))))
-                        (fn [k] (fn ([acc] acc) ([acc v] (rf acc (pair k v))))))
-                      (constantly (multiplexable rf)))
-            m (volatile! (transient {}))]
-        (if (and (= key' kfn) (= val' vfn))
-          (kvrf self
-            ([] (rf))
-            ([acc] (rf (clj/reduce (fn [acc krf] (krf acc)) acc (vals (persistent! @m)))))
-            ([acc x]
-              (self acc (key' x) (val' x)))
-            ([acc k v]
-              (let [krf (or (get @m k) (doto (xform (make-rf k)) (->> (vswap! m assoc! k))))
-                    acc (krf acc v)]
-                 (when (reduced? acc) ; complete?
-                   (vswap! m assoc! k noprf))
-                 (unreduced acc))))
-          (fn
-            ([] (rf))
-            ([acc] (rf (clj/reduce (fn [acc krf] (krf acc)) acc (vals (persistent! @m)))))
-            ([acc x]
-              (let [k (kfn x)
-                    krf (or (get @m k) (doto (xform (make-rf k)) (->> (vswap! m assoc! k))))
-                    acc (krf acc (vfn x))]
-                (when (reduced? acc) ; complete?
-                  (vswap! m assoc! k noprf))
-                (unreduced acc)))))))))
+    (let [pair (if (identical? vector pair) ::default pair)]
+      (fn [rf]
+        (let [make-rf (cond
+                        (nil? pair) (constantly (multiplexable rf))
+                        (= ::default pair)
+                        (if-some [rf (some-kvrf rf)]
+                          (fn [k] (fn ([acc] acc) ([acc v] (rf acc k v))))
+                          (fn [k] (fn ([acc] acc) ([acc v] (rf acc (vector k v))))))
+                        :else (fn [k] (fn ([acc] acc) ([acc v] (rf acc (pair k v))))))
+              m (volatile! (transient {}))]
+          (if (and (nil? kfn) (nil? vfn))
+            (kvrf self
+              ([] (rf))
+              ([acc] (rf (clj/reduce (fn [acc krf] (krf acc)) acc (vals (persistent! @m)))))
+              ([acc x]
+                (self acc (key' x) (val' x)))
+              ([acc k v]
+                (let [krf (or (get @m k) (doto (xform (make-rf k)) (->> (vswap! m assoc! k))))
+                      acc (krf acc v)]
+                   (when (reduced? acc) ; complete?
+                     (vswap! m assoc! k noprf))
+                   (unreduced acc))))
+            (let [kfn (or kfn key')
+                  vfn (or vfn val')]
+              (fn
+                ([] (rf))
+                ([acc] (rf (clj/reduce (fn [acc krf] (krf acc)) acc (vals (persistent! @m)))))
+                ([acc x]
+                  (let [k (kfn x)
+                        krf (or (get @m k) (doto (xform (make-rf k)) (->> (vswap! m assoc! k))))
+                        acc (krf acc (vfn x))]
+                    (when (reduced? acc) ; complete?
+                      (vswap! m assoc! k noprf))
+                    (unreduced acc)))))))))))
 
 (defn- spawn
   "Every n items, spawns a new pipeline."