diff --git a/lib/hdprivatekey.js b/lib/hdprivatekey.js
index a5db864..7cc7e3f 100644
--- a/lib/hdprivatekey.js
+++ b/lib/hdprivatekey.js
@@ -21,7 +21,7 @@ var BufferUtil = require('./util/buffer');
 var JSUtil = require('./util/js');
 
 var MINIMUM_ENTROPY_BITS = 128;
-var BITS_TO_BYTES = 1/8;
+var BITS_TO_BYTES = 1 / 8;
 var MAXIMUM_ENTROPY_BITS = 512;
 
 
@@ -106,18 +106,23 @@ HDPrivateKey._getDerivationIndexes = function(path) {
   }
 
   var indexes = steps.slice(1).map(function(step) {
-    var index = step ? +step : NaN;
-
-    var isHardened = isNaN(index) && step[step.length-1] == "'";
+    var isHardened = step.slice(-1) === '\'';
     if (isHardened) {
-      index = (+(step.slice(0, -1))) + HDPrivateKey.Hardened;
+      step = step.slice(0, -1);
+    }
+    if (!step || step[0] === '-') {
+      return NaN;
+    }
+    var index = +step; // cast to number
+    if (isHardened) {
+      index += HDPrivateKey.Hardened;
     }
 
     return index;
   });
 
   return _.any(indexes, isNaN) ? null : indexes;
-}
+};
 
 /**
  * Get a derivated child based on a string or number.
@@ -177,10 +182,14 @@ HDPrivateKey.prototype._deriveWithNumber = function(index, hardened) {
     data = BufferUtil.concat([this.publicKey.toBuffer(), indexBuffer]);
   }
   var hash = Hash.sha512hmac(data, this._buffers.chainCode);
-  var leftPart = BN.fromBuffer(hash.slice(0, 32), {size: 32});
+  var leftPart = BN.fromBuffer(hash.slice(0, 32), {
+    size: 32
+  });
   var chainCode = hash.slice(32, 64);
 
-  var privateKey = leftPart.add(this.privateKey.toBigNumber()).mod(Point.getN()).toBuffer({size: 32});
+  var privateKey = leftPart.add(this.privateKey.toBigNumber()).mod(Point.getN()).toBuffer({
+    size: 32
+  });
 
   var derived = new HDPrivateKey({
     network: this.network,
@@ -295,7 +304,7 @@ HDPrivateKey.prototype._buildFromSerialized = function(arg) {
     version: decoded.slice(HDPrivateKey.VersionStart, HDPrivateKey.VersionEnd),
     depth: decoded.slice(HDPrivateKey.DepthStart, HDPrivateKey.DepthEnd),
     parentFingerPrint: decoded.slice(HDPrivateKey.ParentFingerPrintStart,
-                                     HDPrivateKey.ParentFingerPrintEnd),
+      HDPrivateKey.ParentFingerPrintEnd),
     childIndex: decoded.slice(HDPrivateKey.ChildIndexStart, HDPrivateKey.ChildIndexEnd),
     chainCode: decoded.slice(HDPrivateKey.ChainCodeStart, HDPrivateKey.ChainCodeEnd),
     privateKey: decoded.slice(HDPrivateKey.PrivateKeyStart, HDPrivateKey.PrivateKeyEnd),
@@ -508,20 +517,20 @@ HDPrivateKey.CheckSumSize = 4;
 HDPrivateKey.DataLength = 78;
 HDPrivateKey.SerializedByteSize = 82;
 
-HDPrivateKey.VersionStart           = 0;
-HDPrivateKey.VersionEnd             = HDPrivateKey.VersionStart + HDPrivateKey.VersionSize;
-HDPrivateKey.DepthStart             = HDPrivateKey.VersionEnd;
-HDPrivateKey.DepthEnd               = HDPrivateKey.DepthStart + HDPrivateKey.DepthSize;
+HDPrivateKey.VersionStart = 0;
+HDPrivateKey.VersionEnd = HDPrivateKey.VersionStart + HDPrivateKey.VersionSize;
+HDPrivateKey.DepthStart = HDPrivateKey.VersionEnd;
+HDPrivateKey.DepthEnd = HDPrivateKey.DepthStart + HDPrivateKey.DepthSize;
 HDPrivateKey.ParentFingerPrintStart = HDPrivateKey.DepthEnd;
-HDPrivateKey.ParentFingerPrintEnd   = HDPrivateKey.ParentFingerPrintStart + HDPrivateKey.ParentFingerPrintSize;
-HDPrivateKey.ChildIndexStart        = HDPrivateKey.ParentFingerPrintEnd;
-HDPrivateKey.ChildIndexEnd          = HDPrivateKey.ChildIndexStart + HDPrivateKey.ChildIndexSize;
-HDPrivateKey.ChainCodeStart         = HDPrivateKey.ChildIndexEnd;
-HDPrivateKey.ChainCodeEnd           = HDPrivateKey.ChainCodeStart + HDPrivateKey.ChainCodeSize;
-HDPrivateKey.PrivateKeyStart        = HDPrivateKey.ChainCodeEnd + 1;
-HDPrivateKey.PrivateKeyEnd          = HDPrivateKey.PrivateKeyStart + HDPrivateKey.PrivateKeySize;
-HDPrivateKey.ChecksumStart          = HDPrivateKey.PrivateKeyEnd;
-HDPrivateKey.ChecksumEnd            = HDPrivateKey.ChecksumStart + HDPrivateKey.CheckSumSize;
+HDPrivateKey.ParentFingerPrintEnd = HDPrivateKey.ParentFingerPrintStart + HDPrivateKey.ParentFingerPrintSize;
+HDPrivateKey.ChildIndexStart = HDPrivateKey.ParentFingerPrintEnd;
+HDPrivateKey.ChildIndexEnd = HDPrivateKey.ChildIndexStart + HDPrivateKey.ChildIndexSize;
+HDPrivateKey.ChainCodeStart = HDPrivateKey.ChildIndexEnd;
+HDPrivateKey.ChainCodeEnd = HDPrivateKey.ChainCodeStart + HDPrivateKey.ChainCodeSize;
+HDPrivateKey.PrivateKeyStart = HDPrivateKey.ChainCodeEnd + 1;
+HDPrivateKey.PrivateKeyEnd = HDPrivateKey.PrivateKeyStart + HDPrivateKey.PrivateKeySize;
+HDPrivateKey.ChecksumStart = HDPrivateKey.PrivateKeyEnd;
+HDPrivateKey.ChecksumEnd = HDPrivateKey.ChecksumStart + HDPrivateKey.CheckSumSize;
 
 assert(HDPrivateKey.ChecksumEnd === HDPrivateKey.SerializedByteSize);
 
diff --git a/test/hdprivatekey.js b/test/hdprivatekey.js
index 0888007..4a0d20b 100644
--- a/test/hdprivatekey.js
+++ b/test/hdprivatekey.js
@@ -14,7 +14,6 @@ var Base58Check = bitcore.encoding.Base58Check;
 
 var xprivkey = 'xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi';
 var json = '{"network":"livenet","depth":0,"fingerPrint":876747070,"parentFingerPrint":0,"childIndex":0,"chainCode":"873dff81c02f525623fd1fe5167eac3a55a049de3d314bb42ee227ffed37d508","privateKey":"e8f32e723decf4051aefac8e2c93c9c5b214313817cdb01a1494b917c8436b35","checksum":-411132559,"xprivkey":"xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi"}';
-
 describe('HDPrivate key interface', function() {
   /* jshint maxstatements: 50 */
   var expectFail = function(func, error) {
@@ -113,36 +112,33 @@ describe('HDPrivate key interface', function() {
     derivedByNumber.xprivkey.should.equal(derivedByArgument.xprivkey);
   });
 
-  it('returns itself with "m" parameter', function() {
+  it('returns itself with \'m\' parameter', function() {
     var privateKey = new HDPrivateKey(xprivkey);
     privateKey.should.equal(privateKey.derive('m'));
   });
 
   it('returns InvalidArgument if invalid data is given to getSerializedError', function() {
     expect(
-      HDPrivateKey.getSerializedError(1) instanceof
-      hdErrors.UnrecognizedArgument
+      HDPrivateKey.getSerializedError(1) instanceof hdErrors.UnrecognizedArgument
     ).to.equal(true);
   });
 
   it('returns InvalidLength if data of invalid length is given to getSerializedError', function() {
+    var b58s = Base58Check.encode(new buffer.Buffer('onestring'));
     expect(
-      HDPrivateKey.getSerializedError(Base58Check.encode(new buffer.Buffer('onestring'))) instanceof
-      hdErrors.InvalidLength
+      HDPrivateKey.getSerializedError(b58s) instanceof hdErrors.InvalidLength
     ).to.equal(true);
   });
 
   it('returns InvalidNetworkArgument if an invalid network is provided', function() {
     expect(
-      HDPrivateKey.getSerializedError(xprivkey, 'invalidNetwork') instanceof
-      errors.InvalidNetworkArgument
+      HDPrivateKey.getSerializedError(xprivkey, 'invalidNetwork') instanceof errors.InvalidNetworkArgument
     ).to.equal(true);
   });
 
   it('recognizes that the wrong network was asked for', function() {
     expect(
-      HDPrivateKey.getSerializedError(xprivkey, 'testnet') instanceof
-      errors.InvalidNetwork
+      HDPrivateKey.getSerializedError(xprivkey, 'testnet') instanceof errors.InvalidNetwork
     ).to.equal(true);
   });
 
@@ -200,7 +196,7 @@ describe('HDPrivate key interface', function() {
     it('validates correct paths', function() {
       var valid;
 
-      valid = HDPrivateKey.isValidPath("m/0'/1/2'");
+      valid = HDPrivateKey.isValidPath('m/0\'/1/2\'');
       valid.should.equal(true);
 
       valid = HDPrivateKey.isValidPath('m');
@@ -219,72 +215,50 @@ describe('HDPrivate key interface', function() {
       valid.should.equal(true);
     });
 
-    it('rejects illegal paths', function() {
-      var valid;
 
-      valid = HDPrivateKey.isValidPath('m/-1/12');
-      valid.should.equal(false);
+    var invalid = [
+      'm/-1/12',
+      'bad path',
+      'K',
+      'm/',
+      'm/12asd',
+      'm/1/2//3'
+    ];
 
-      valid = HDPrivateKey.isValidPath('bad path');
-      valid.should.equal(false);
-
-      valid = HDPrivateKey.isValidPath('K');
-      valid.should.equal(false);
-
-      valid = HDPrivateKey.isValidPath('m/');
-      valid.should.equal(false);
-
-      valid = HDPrivateKey.isValidPath('m/12asd');
-      valid.should.equal(false);
-
-      valid = HDPrivateKey.isValidPath(HDPrivateKey.MaxHardened);
-      valid.should.equal(false);
+    invalid.forEach(function(datum) {
+      it('rejects illegal path ' + datum, function() {
+        HDPrivateKey.isValidPath(datum).should.equal(false);
+        expect(HDPrivateKey._getDerivationIndexes(datum)).to.equal(null);
+      });
     });
 
     it('generates deriving indexes correctly', function() {
       var indexes;
 
       indexes = HDPrivateKey._getDerivationIndexes('m/-1/12');
-      indexes.should.eql([-1, 12]);
+      expect(indexes).to.equal(null);
 
-      indexes = HDPrivateKey._getDerivationIndexes("m/0/12/12'");
+      indexes = HDPrivateKey._getDerivationIndexes('m/0/12/12\'');
       indexes.should.eql([0, 12, HDPrivateKey.Hardened + 12]);
 
-      indexes = HDPrivateKey._getDerivationIndexes("m/0/12/12'");
+      indexes = HDPrivateKey._getDerivationIndexes('m/0/12/12\'');
       indexes.should.eql([0, 12, HDPrivateKey.Hardened + 12]);
     });
 
-    it('rejects invalid derivation path', function() {
-      var indexes;
-
-      indexes = HDPrivateKey._getDerivationIndexes("m/");
-      expect(indexes).to.be.null;
-
-      indexes = HDPrivateKey._getDerivationIndexes("bad path");
-      expect(indexes).to.be.null;
-
-      indexes = HDPrivateKey._getDerivationIndexes("K");
-      expect(indexes).to.be.null;
-
-      indexes = HDPrivateKey._getDerivationIndexes("m/");
-      expect(indexes).to.be.null;
-
-      indexes = HDPrivateKey._getDerivationIndexes("m/123asd");
-      expect(indexes).to.be.null;
-    });
   });
 
   describe('conversion to plain object/json', function() {
     var plainObject = {
-      'network':'livenet',
-      'depth':0,
-      'fingerPrint':876747070,
-      'parentFingerPrint':0,
-      'childIndex':0,
-      'chainCode':'873dff81c02f525623fd1fe5167eac3a55a049de3d314bb42ee227ffed37d508',
-      'privateKey':'e8f32e723decf4051aefac8e2c93c9c5b214313817cdb01a1494b917c8436b35',
-      'checksum':-411132559,
-      'xprivkey':'xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi'
+      'network': 'livenet',
+      'depth': 0,
+      'fingerPrint': 876747070,
+      'parentFingerPrint': 0,
+      'childIndex': 0,
+      'chainCode': '873dff81c02f525623fd1fe5167eac3a55a049de3d314bb42ee227ffed37d508',
+      'privateKey': 'e8f32e723decf4051aefac8e2c93c9c5b214313817cdb01a1494b917c8436b35',
+      'checksum': -411132559,
+      'xprivkey': 'xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvN' +
+        'KmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi'
     };
     it('toObject leaves no Buffer instances', function() {
       var privKey = new HDPrivateKey(xprivkey);
@@ -308,4 +282,3 @@ describe('HDPrivate key interface', function() {
     });
   });
 });
-