diff --git a/lib/unique-message-id-provider.js b/lib/unique-message-id-provider.js
index 5d2203f14..6ffd4bde6 100644
--- a/lib/unique-message-id-provider.js
+++ b/lib/unique-message-id-provider.js
@@ -18,7 +18,8 @@ function UniqueMessageIdProvider () {
  * allocate
  *
  * Get the next messageId.
- * @return unsigned int
+ * @return if messageId is fully allocated then return null,
+ *         otherwise return the smallest usable unsigned int messageId.
  */
 UniqueMessageIdProvider.prototype.allocate = function () {
   this.lastId = this.numberAllocator.alloc()