diff --git a/lib/hd/private.js b/lib/hd/private.js
index 89c381a9..25e4f4ac 100644
--- a/lib/hd/private.js
+++ b/lib/hd/private.js
@@ -177,22 +177,25 @@ HDPrivateKey.prototype.destroy = function destroy(pub) {
 
 /**
  * Derive a child key.
- * @param {Number|String} - Child index or path.
+ * @param {Number} index - Derivation index.
  * @param {Boolean?} hardened - Whether the derivation should be hardened.
  * @returns {HDPrivateKey}
  */
 
 HDPrivateKey.prototype.derive = function derive(index, hardened) {
-  assert(util.isU32(index));
+  assert(typeof index === 'number');
+
+  if ((index >>> 0) !== index)
+    throw new Error('Index out of range.');
+
+  if (this.depth >= 0xff)
+    throw new Error('Depth too high.');
 
   if (hardened) {
     index |= common.HARDENED;
     index >>>= 0;
   }
 
-  if (this.depth >= 0xff)
-    throw new Error('Depth too high.');
-
   const id = this.getID(index);
   const cache = common.cache.get(id);
 
diff --git a/lib/hd/public.js b/lib/hd/public.js
index 3ce7772d..6ff97a97 100644
--- a/lib/hd/public.js
+++ b/lib/hd/public.js
@@ -143,7 +143,7 @@ HDPublicKey.prototype.destroy = function destroy() {
 
 /**
  * Derive a child key.
- * @param {Number|String} - Child index or path.
+ * @param {Number} index - Derivation index.
  * @param {Boolean?} hardened - Whether the derivation
  * should be hardened (throws if true).
  * @returns {HDPrivateKey}
@@ -151,7 +151,10 @@ HDPublicKey.prototype.destroy = function destroy() {
  */
 
 HDPublicKey.prototype.derive = function derive(index, hardened) {
-  assert(util.isU32(index));
+  assert(typeof index === 'number');
+
+  if ((index >>> 0) !== index)
+    throw new Error('Index out of range.');
 
   if ((index & common.HARDENED) || hardened)
     throw new Error('Cannot derive hardened.');