diff --git a/lib/bcoin/hd.js b/lib/bcoin/hd.js
index c4f3770f..d6395802 100644
--- a/lib/bcoin/hd.js
+++ b/lib/bcoin/hd.js
@@ -77,6 +77,8 @@ function HDSeed(options) {
   this.entropy = options.entropy;
   this.mnemonic = options.mnemonic;
   this.passphrase = options.passphrase || '';
+
+  assert(this.bits % 8 === 0);
 }
 
 HDSeed.prototype.createSeed = function createSeed(passphrase) {
@@ -126,6 +128,8 @@ HD.fromSeed = function fromSeed(options) {
   return HDPrivateKey.fromSeed(options);
 };
 
+HD.cache = new bcoin.lru(500);
+
 /**
  * HD Private Key
  */
@@ -719,7 +723,7 @@ HDPrivateKey.prototype.derive = function derive(index, hardened) {
   if (typeof index === 'string')
     return this.derivePath(index);
 
-  cached = cache.get(this.xprivkey, index);
+  cached = HD.cache.get(this.xprivkey + '/' + index);
 
   if (cached)
     return cached;
@@ -766,7 +770,7 @@ HDPrivateKey.prototype.derive = function derive(index, hardened) {
     }
   });
 
-  cache.set(this.xprivkey, index, child);
+  HD.cache.set(this.xprivkey + '/' + index, child);
 
   return child;
 };
@@ -1087,7 +1091,7 @@ HDPublicKey.prototype.derive = function derive(index, hardened) {
   if (typeof index === 'string')
     return this.derivePath(index);
 
-  cached = cache.get(this.xpubkey, index);
+  cached = HD.cache.get(this.xpubkey + '/' + index);
 
   if (cached)
     return cached;
@@ -1128,7 +1132,7 @@ HDPublicKey.prototype.derive = function derive(index, hardened) {
     }
   });
 
-  cache.set(this.xpubkey, index, child);
+  HD.cache.set(this.xpubkey + '/' + index, child);
 
   return child;
 };
@@ -1263,34 +1267,6 @@ function pbkdf2(key, salt, iterations, dkLen) {
   return DK;
 }
 
-HD.cache = new bcoin.lru(500, function(key, value) {
-  return 1;
-});
-
-var cache = {
-  data: {},
-  count: 0
-};
-
-cache.set = function(key, index, value) {
-  key = key + '/' + index;
-
-  if (this.count > 500) {
-    this.data = {};
-    this.count = 0;
-  }
-
-  if (this.data[key] === undefined)
-    this.count++;
-
-  this.data[key] = value;
-};
-
-cache.get = function(key, index) {
-  key = key + '/' + index;
-  return this.data[key];
-};
-
 /**
  * Expose
  */