diff --git a/src/derivers/bip32.ts b/src/derivers/bip32.ts
index 05f009fc..c3f839fb 100755
--- a/src/derivers/bip32.ts
+++ b/src/derivers/bip32.ts
@@ -29,11 +29,12 @@ export function bip32PathToMultipath(bip32Path: string): string {
   return multipath;
 }
 
-export function deriveChildKey(parentKey: Buffer, pathPart: string): Buffer {
+export function deriveChildKey(pathPart: string, parentKey: Buffer): Buffer {
   const isHardened = pathPart.includes(`'`);
   const indexPart = pathPart.split(`'`)[0];
   const childIndex = parseInt(indexPart, 10);
   assert(childIndex < HARDENED_OFFSET, 'Invalid index');
+  assert(Boolean(parentKey), 'Must provide parentKey');
   assert(parentKey.length === 64, 'Parent key invalid length');
 
   const parentPrivateKey = parentKey.slice(0, 32);
diff --git a/src/derivers/bip39.ts b/src/derivers/bip39.ts
index 064738d4..7db180c6 100755
--- a/src/derivers/bip39.ts
+++ b/src/derivers/bip39.ts
@@ -8,7 +8,7 @@ export function bip39MnemonicToMultipath(mnemonic: string): string {
 }
 
 // this creates a child key using bip39, ignoring the parent key
-export function deriveChildKey(_parentKey: unknown, pathPart: string): Buffer {
+export function deriveChildKey(pathPart: string, _parentKey?: never): Buffer {
   const mnemonic = pathPart;
   const seedBuffer = bip39.mnemonicToSeed(mnemonic);
   const entropy = crypto.createHmac('sha512', ROOT_BASE_SECRET).update(seedBuffer).digest();
diff --git a/src/derivers/index.ts b/src/derivers/index.ts
index ff4b4daf..9cffa1e6 100644
--- a/src/derivers/index.ts
+++ b/src/derivers/index.ts
@@ -1,7 +1,11 @@
 import * as bip32 from './bip32';
 import * as bip39 from './bip39';
 
-export = {
+export interface Deriver {
+  deriveChildKey: (pathPart: string, parentKey?: Buffer) => Buffer;
+}
+
+export const derivers = {
   bip32,
   bip39,
 };
diff --git a/src/index.ts b/src/index.ts
index 02d1e06a..fe6c3448 100755
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,6 +1,11 @@
 import bip39 from 'bip39';
-import derivers from './derivers';
+import { derivers, Deriver } from './derivers';
 
+/**
+ * Converts the given BIP-39 mnemonic to a cryptographic seed.
+ * @param mnemonic - The BIP-39 mnemonic.
+ * @returns The cryptographic seed corresponding to the given mnemonic.
+ */
 export function mnemonicToSeed(mnemonic: string): Buffer {
   return bip39.mnemonicToSeed(mnemonic);
 }
@@ -18,11 +23,26 @@ export function mnemonicToSeed(mnemonic: string): Buffer {
  */
 
 /**
- * @param {string} pathSegment - A full or leaf HD path segment. If full,
- * optionally preceded by "bip39:<SPACE_DELIMITED_SEED_PHRASE>/".
- * @param {Buffer} [parentKey] - The parent key of the given path segment.
+ * Takes a full or partial HD path string and returns the key corresponding to
+ * the given path, with the following constraints:
+ *
+ * - If the path starts with a BIP-32 segment, a parent key must be provided.
+ * - If the path starts with a BIP-39 segment, a parent key may NOT be provided.
+ * - The path cannot exceed 5 BIP-32 segments in length, optionally preceded by
+ *   a single BIP-39 segment.
+ *
+ * WARNING: It is the consumer's responsibility to ensure that the path is valid
+ * relative to its parent key.
+ *
+ * @param pathSegment - A full or partial HD path, e.g.:
+ * bip39:SEED_PHRASE/bip32:44'/bip32:60'/bip32:0'/bip32:0/bip32:0
+ *
+ * BIP-39 seed phrases must be lowercase, space-delimited, and 12-24 words long.
+ *
+ * @param parentKey - The parent key of the given path segment, if any.
+ * @returns The derived key.
  */
-export function deriveKeyFromPath(pathSegment: string, parentKey: Buffer) {
+export function deriveKeyFromPath(pathSegment: string, parentKey?: Buffer): Buffer {
   validateDeriveKeyParams(pathSegment, parentKey);
 
   let key = parentKey;
@@ -33,13 +53,13 @@ export function deriveKeyFromPath(pathSegment: string, parentKey: Buffer) {
     if (!(hasDeriver(pathType))) {
       throw new Error(`Unknown derivation type "${pathType}"`);
     }
-    const deriver = derivers[pathType];
-    const childKey = deriver.deriveChildKey(key, pathValue);
+    const deriver = derivers[pathType] as Deriver;
+    const childKey = deriver.deriveChildKey(pathValue, key);
     // continue deriving from child key
     key = childKey;
   });
 
-  return key;
+  return key as Buffer;
 }
 
 function hasDeriver(pathType: string): pathType is keyof typeof derivers {
@@ -67,7 +87,7 @@ const BIP_39_PATH_REGEX = /^bip39:([a-z]+){1}( [a-z]+){11,23}$/u;
  */
 const MULTI_PATH_REGEX = /^(bip39:([a-z]+){1}( [a-z]+){11,23}\/)?(bip32:\d+'?\/){0,4}(bip32:\d+'?)$/u;
 
-function validateDeriveKeyParams(pathSegment: string, parentKey: Buffer) {
+function validateDeriveKeyParams(pathSegment: string, parentKey?: Buffer) {
   // The path segment must be one of the following:
   // - A lone BIP-32 path segment
   // - A lone BIP-39 path segment
diff --git a/test/index.js b/test/index.js
index a96701de..d27e554f 100755
--- a/test/index.js
+++ b/test/index.js
@@ -8,7 +8,7 @@ const {
     privateKeyToEthAddress,
   },
   bip39: { deriveChildKey: bip39Derive, bip39MnemonicToMultipath },
-} = require('../dist/derivers');
+} = require('../dist/derivers').derivers;
 
 const defaultEthereumPath = `m/44'/60'/0'/0`;
 
@@ -121,15 +121,15 @@ test('bip32Derive', (t) => {
   // generate parent key
   let parentKey = null;
   parentKey = bip39Derive(
-    parentKey,
     `romance hurry grit huge rifle ordinary loud toss sound congress upset twist`,
+    parentKey,
   );
-  parentKey = bip32Derive(parentKey, `44'`);
-  parentKey = bip32Derive(parentKey, `60'`);
-  parentKey = bip32Derive(parentKey, `0'`);
-  parentKey = bip32Derive(parentKey, `0`);
+  parentKey = bip32Derive(`44'`, parentKey);
+  parentKey = bip32Derive(`60'`, parentKey);
+  parentKey = bip32Derive(`0'`, parentKey);
+  parentKey = bip32Derive(`0`, parentKey);
   const keys = expectedAddresses.map((_, index) => {
-    return bip32Derive(parentKey, `${index}`);
+    return bip32Derive(`${index}`, parentKey);
   });
   // validate addresses
   keys.forEach((key, index) => {