diff --git a/src/hdwallet.js b/src/hdwallet.js
index 20766ac..7df6207 100644
--- a/src/hdwallet.js
+++ b/src/hdwallet.js
@@ -12,14 +12,11 @@ var networks = require('./networks')
 var sec = require('./sec')
 var ecparams = sec("secp256k1")
 
-function HDWallet(seed, networkString) {
+function HDWallet(seed, network) {
   if (seed == undefined) return; // FIXME: Boo, should be stricter
 
-  this.network = networkString || 'bitcoin'
-
-  if(!networks.hasOwnProperty(this.network)) {
-    throw new Error("Unknown network: " + this.network)
-  }
+  network = network || networks.bitcoin
+  assert(network.bip32, 'Unknown BIP32 constants for network')
 
   var I = crypto.HmacSHA512(seed, HDWallet.MASTER_SECRET)
   var IL = I.slice(0, 32)
@@ -28,6 +25,7 @@ function HDWallet(seed, networkString) {
   // In case IL is 0 or >= n, the master key is invalid (handled by ECKey.fromBuffer)
   var pIL = BigInteger.fromBuffer(IL)
 
+  this.network = network
   this.priv = new ECKey(pIL, true)
   this.pub = this.priv.pub
 
@@ -40,8 +38,8 @@ HDWallet.MASTER_SECRET = new Buffer('Bitcoin seed')
 HDWallet.HIGHEST_BIT = 0x80000000
 HDWallet.LENGTH = 78
 
-HDWallet.fromSeedHex = function(hex, networkString) {
-  return new HDWallet(new Buffer(hex, 'hex'), networkString)
+HDWallet.fromSeedHex = function(hex, network) {
+  return new HDWallet(new Buffer(hex, 'hex'), network)
 }
 
 HDWallet.fromBase58 = function(string) {
@@ -66,14 +64,14 @@ HDWallet.fromBuffer = function(input) {
   var version = input.readUInt32BE(0)
 
   var type
-  for(var name in networks) {
+  for (var name in networks) {
     var network = networks[name]
 
-    for(var t in network.bip32) {
+    for (var t in network.bip32) {
       if (version != network.bip32[t]) continue
 
       type = t
-      hd.network = name
+      hd.network = network
     }
   }
 
@@ -127,7 +125,7 @@ HDWallet.prototype.getAddress = function() {
 
 HDWallet.prototype.toBuffer = function(priv) {
   // Version
-  var version = networks[this.network].bip32[priv ? 'priv' : 'pub']
+  var version = this.network.bip32[priv ? 'priv' : 'pub']
   var buffer = new Buffer(HDWallet.LENGTH)
 
   // 4 bytes: version bytes
@@ -254,7 +252,7 @@ HDWallet.prototype.derivePrivate = function(index) {
 }
 
 HDWallet.prototype.getKeyVersion = function() {
-  return networks[this.network].pubKeyHash
+  return this.network.pubKeyHash
 }
 
 HDWallet.prototype.toString = HDWallet.prototype.toBase58
diff --git a/src/wallet.js b/src/wallet.js
index 206bf44..58b5748 100644
--- a/src/wallet.js
+++ b/src/wallet.js
@@ -1,14 +1,14 @@
-var Address = require('./address')
-var HDNode = require('./hdwallet.js')
 var networks = require('./networks')
 var rng = require('secure-random')
+
+var Address = require('./address')
+var HDNode = require('./hdwallet')
 var Transaction = require('./transaction').Transaction
 
-function Wallet(seed, options) {
+function Wallet(seed, network) {
   if (!(this instanceof Wallet)) { return new Wallet(seed, options); }
 
-  var options = options || {}
-  var network = options.network || 'bitcoin'
+  network = network || networks.bitcoin
 
   // Stored in a closure to make accidental serialization less likely
   var masterkey = null
@@ -28,7 +28,7 @@ function Wallet(seed, options) {
   this.outputs = {}
 
   // Make a new master key
-  this.newMasterKey = function(seed, network) {
+  this.newMasterKey = function(seed) {
     seed = seed || new Buffer(rng(32))
     masterkey = new HDNode(seed, network)
 
@@ -43,7 +43,8 @@ function Wallet(seed, options) {
 
     me.outputs = {}
   }
-  this.newMasterKey(seed, network)
+
+  this.newMasterKey(seed)
 
   this.generateAddress = function() {
     var key = externalAccount.derive(this.addresses.length)
@@ -143,7 +144,7 @@ function Wallet(seed, options) {
       var address
 
       try {
-        address = Address.fromScriptPubKey(txOut.script, networks[network]).toString()
+        address = Address.fromScriptPubKey(txOut.script, network).toString()
       } catch(e) {
         if (!(e.message.match(/has no matching Address/))) throw e
       }
diff --git a/test/hdwallet.js b/test/hdwallet.js
index 0a826c7..03a7c85 100644
--- a/test/hdwallet.js
+++ b/test/hdwallet.js
@@ -1,4 +1,5 @@
 var assert = require('assert')
+var networks = require('../src/networks')
 
 var HDWallet = require('../src/hdwallet')
 var fixtures = require('./fixtures/hdwallet.json')
@@ -68,6 +69,15 @@ describe('HDWallet', function() {
         buffer.writeUInt32BE(0xFFFFFFFF, 9)
         assert.throws(function() { HDWallet.fromBuffer(buffer) }, /Invalid index/)
       })
+
+      it('fails for an invalid network type', function() {
+        var buffer = new HDWallet(seed).toBuffer()
+        buffer.writeUInt32BE(0x00000000, 0)
+
+        assert.throws(function() {
+          HDWallet.fromBuffer(buffer)
+        }, /Could not find version 0/)
+      })
     })
   })
 
@@ -119,24 +129,36 @@ describe('HDWallet', function() {
   })
 
   describe('network types', function() {
+    var seed
+
+    beforeEach(function() {
+      seed = new Buffer('foobar')
+    })
+
+    it('ensure that a bitcoin wallet is the default', function() {
+      var wallet = new HDWallet(seed)
+
+      assert.equal(wallet.network, networks.bitcoin)
+    })
+
     it('ensures that a bitcoin Wallet generates bitcoin addresses', function() {
-      var wallet = new HDWallet(new Buffer('foobar'), 'bitcoin')
-      assert.equal(wallet.getAddress().toString(), '17SnB9hyGwJPoKpLb9eVPHjsujyEuBpMAA')
+      var wallet = new HDWallet(seed)
+      var address = wallet.getAddress().toString()
+
+      assert.equal(address, '17SnB9hyGwJPoKpLb9eVPHjsujyEuBpMAA')
     })
 
     it('ensures that a testnet Wallet generates testnet addresses', function() {
-      var wallet = new HDWallet(new Buffer('foobar'), 'testnet')
-      assert.equal(wallet.getAddress().toString(), 'mmxjUCnx5xjeaSHxJicsDCxCmjZwq8KTbv')
-    })
+      var wallet = new HDWallet(seed, networks.testnet)
+      var address = wallet.getAddress().toString()
 
-    it('throws an exception when unknown network type is passed in', function() {
-      assert.throws(function() { new HDWallet(new Buffer('foobar'), 'doge') }, /Unknown network: doge/)
+      assert.equal(address, 'mmxjUCnx5xjeaSHxJicsDCxCmjZwq8KTbv')
     })
 
-    it('throws an exception with bad network type using fromBuffer', function() {
-      var buffer = new HDWallet(new Buffer('foobar'), 'bitcoin').toBuffer()
-      buffer.writeUInt32BE(0x00000000, 0)
-      assert.throws(function() { HDWallet.fromBuffer(buffer) }, /Could not find version 0/)
+    it('throws an exception when unknown network type is passed in', function() {
+      assert.throws(function() {
+        new HDWallet(seed, {})
+      }, /Unknown BIP32 constants for network/)
     })
   })
 })
diff --git a/test/wallet.js b/test/wallet.js
index b5187d1..e54d603 100644
--- a/test/wallet.js
+++ b/test/wallet.js
@@ -1,5 +1,6 @@
 var assert = require('assert')
 var crypto = require('../src/crypto')
+var networks = require('../src/networks')
 var sinon = require('sinon')
 
 var Address = require('../src/address')
@@ -29,7 +30,7 @@ describe('Wallet', function() {
     })
 
     it('defaults to Bitcoin network', function() {
-      assert.equal(wallet.getMasterKey().network, 'bitcoin')
+      assert.equal(wallet.getMasterKey().network, networks.bitcoin)
     })
 
     it("generates m/0' as the main account", function() {
@@ -59,11 +60,11 @@ describe('Wallet', function() {
 
     describe('constructor options', function() {
       beforeEach(function() {
-        wallet = new Wallet(seed, {network: 'testnet'})
+        wallet = new Wallet(seed, networks.testnet)
       })
 
       it('uses the network if specified', function() {
-        assert.equal(wallet.getMasterKey().network, 'testnet')
+        assert.equal(wallet.getMasterKey().network, networks.testnet)
       })
     })
   })
@@ -98,7 +99,7 @@ describe('Wallet', function() {
 
   describe('generateAddress', function(){
     it('generate receiving addresses', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
       var expectedAddresses = [
         "n1GyUANZand9Kw6hGSV9837cCC9FFUQzQa",
         "n2fiWrHqD6GM5GiEqkbWAc6aaZQp3ba93X"
@@ -112,7 +113,7 @@ describe('Wallet', function() {
 
   describe('generateChangeAddress', function(){
     it('generates change addresses', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
       var expectedAddresses = ["mnXiDR4MKsFxcKJEZjx4353oXvo55iuptn"]
 
       assert.equal(wallet.generateChangeAddress(), expectedAddresses[0])
@@ -122,7 +123,7 @@ describe('Wallet', function() {
 
   describe('getPrivateKey', function(){
     it('returns the private key at the given index of external account', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
 
       assertEqual(wallet.getPrivateKey(0), wallet.getExternalAccount().derive(0).priv)
       assertEqual(wallet.getPrivateKey(1), wallet.getExternalAccount().derive(1).priv)
@@ -131,7 +132,7 @@ describe('Wallet', function() {
 
   describe('getInternalPrivateKey', function(){
     it('returns the private key at the given index of internal account', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
 
       assertEqual(wallet.getInternalPrivateKey(0), wallet.getInternalAccount().derive(0).priv)
       assertEqual(wallet.getInternalPrivateKey(1), wallet.getInternalAccount().derive(1).priv)
@@ -140,7 +141,7 @@ describe('Wallet', function() {
 
   describe('getPrivateKeyForAddress', function(){
     it('returns the private key for the given address', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
       wallet.generateChangeAddress()
       wallet.generateAddress()
       wallet.generateAddress()
@@ -156,7 +157,7 @@ describe('Wallet', function() {
     })
 
     it('raises an error when address is not found', function(){
-      var wallet = new Wallet(seed, {network: 'testnet'})
+      var wallet = new Wallet(seed, networks.testnet)
       assert.throws(function() {
         wallet.getPrivateKeyForAddress("n2fiWrHqD6GM5GiEqkbWAc6aaZQp3ba93X")
       }, /Unknown address. Make sure the address is from the keychain and has been generated./)
@@ -434,9 +435,9 @@ describe('Wallet', function() {
       })
     })
 
-    describe('testnet', function(){
+    describe(networks.testnet, function(){
       it('should create transaction', function(){
-        var wallet = new Wallet(seed, {network: 'testnet'})
+        var wallet = new Wallet(seed, networks.testnet)
         var address = wallet.generateAddress()
 
         wallet.setUnspentOutputs([{
@@ -458,7 +459,7 @@ describe('Wallet', function() {
 
     describe('changeAddress', function(){
       it('should allow custom changeAddress', function(){
-        var wallet = new Wallet(seed, {network: 'testnet'})
+        var wallet = new Wallet(seed, networks.testnet)
         var address = wallet.generateAddress()
 
         wallet.setUnspentOutputs([{