diff --git a/lib/bcoin/lru.js b/lib/bcoin/lru.js
index 397f632c..160b995b 100644
--- a/lib/bcoin/lru.js
+++ b/lib/bcoin/lru.js
@@ -53,7 +53,7 @@ LRU.prototype._createGetSize = function _createGetSize(size) {
 /**
  * Calculate size of an item.
  * @private
- * @param {Object} item
+ * @param {LRUItem} item
  * @returns {Number} Size.
  */
 
@@ -139,6 +139,9 @@ LRU.prototype.reset = function reset() {
 LRU.prototype.set = function set(key, value) {
   var item;
 
+  if (value === undefined)
+    throw new Error('Cannot set value to undefined.');
+
   key = key + '';
 
   item = this.data[key];
@@ -193,7 +196,7 @@ LRU.prototype.get = function get(key) {
  */
 
 LRU.prototype.has = function get(key) {
-  return this.data[key] != null;
+  return this.data[key] !== undefined;
 };
 
 /**
@@ -221,14 +224,33 @@ LRU.prototype.remove = function remove(key) {
   return true;
 };
 
+/**
+ * Prepend an item to the linked list (sets new head).
+ * @private
+ * @param {LRUItem}
+ */
+
 LRU.prototype._prependList = function prependList(item) {
   this._insertList(null, item);
 };
 
+/**
+ * Append an item to the linked list (sets new tail).
+ * @private
+ * @param {LRUItem}
+ */
+
 LRU.prototype._appendList = function appendList(item) {
   this._insertList(this.tail, item);
 };
 
+/**
+ * Insert item into the linked list.
+ * @private
+ * @param {LRUItem|null} ref
+ * @param {LRUItem} item
+ */
+
 LRU.prototype._insertList = function insertList(ref, item) {
   assert(!item.next);
   assert(!item.prev);
@@ -253,6 +275,12 @@ LRU.prototype._insertList = function insertList(ref, item) {
     this.tail = item;
 };
 
+/**
+ * Remove item from the linked list.
+ * @private
+ * @param {LRUItem}
+ */
+
 LRU.prototype._removeList = function removeList(item) {
   if (item.prev)
     item.prev.next = item.next;