diff --git a/src/taoensso/nippy.clj b/src/taoensso/nippy.clj
index 96bc300..9b645a9 100644
--- a/src/taoensso/nippy.clj
+++ b/src/taoensso/nippy.clj
@@ -1426,15 +1426,18 @@
     (.writeUTF [data-output] (:data x)))"
   [type custom-type-id [x out] & body]
   (assert-custom-type-id custom-type-id)
-  `(extend-type ~type IFreezable1
-     (~'-freeze-without-meta! [~x ~(with-meta out {:tag 'java.io.DataOutput})]
-       (if-not ~(keyword? custom-type-id)
-         ;; Unprefixed [cust byte id][payload]:
-         (write-id ~out ~(coerce-custom-type-id custom-type-id))
-         ;; Prefixed [const byte id][cust hash id][payload]:
-         (do (write-id    ~out ~id-prefixed-custom)
-             (.writeShort ~out ~(coerce-custom-type-id custom-type-id))))
-      ~@body)))
+  (let [write-id-form
+        (if (keyword? custom-type-id)
+          ;; Prefixed [const byte id][cust hash id][payload]:
+          `(do (write-id    ~out ~id-prefixed-custom)
+               (.writeShort ~out ~(coerce-custom-type-id custom-type-id)))
+          ;; Unprefixed [cust byte id][payload]:
+          `(write-id ~out ~(coerce-custom-type-id custom-type-id)))]
+
+    `(extend-type ~type IFreezable1
+       (~'-freeze-without-meta! [~x ~(with-meta out {:tag 'java.io.DataOutput})]
+        ~write-id-form
+        ~@body))))
 
 (defmacro extend-thaw
   "Extends Nippy to support thawing of a custom type with given id: