diff --git a/lib/bcoin/mtx.js b/lib/bcoin/mtx.js
index cf8ec2ec..08766213 100644
--- a/lib/bcoin/mtx.js
+++ b/lib/bcoin/mtx.js
@@ -13,6 +13,7 @@ var assert = utils.assert;
 var constants = bcoin.protocol.constants;
 var Script = bcoin.script;
 var opcodes = constants.opcodes;
+var HASH160 = constants.ZERO_HASH.slice(0, 20);
 
 /**
  * A mutable transaction object.
@@ -387,6 +388,8 @@ MTX.prototype.signInput = function signInput(index, addr, key, type) {
   // signing p2sh transactions.
   if (prev.isScripthash()) {
     prev = input.script.getRedeem();
+    if (!prev)
+      throw new Error('Input has not been templated.');
     len = vector.length - 1;
   }
 
@@ -398,6 +401,8 @@ MTX.prototype.signInput = function signInput(index, addr, key, type) {
   // pushes onto the stack).
   if (prev.isWitnessScripthash()) {
     prev = input.witness.getRedeem();
+    if (!prev)
+      throw new Error('Input has not been templated.');
     vector = input.witness;
     len = vector.length - 1;
     version = 1;
@@ -421,6 +426,9 @@ MTX.prototype.signInput = function signInput(index, addr, key, type) {
     if (!utils.equal(addr.publicKey, prev.get(0)))
       return false;
 
+    if (vector.getSmall(0) !== 0)
+      throw new Error('Input has not been templated.');
+
     vector.set(0, signature);
     vector.compile();
 
@@ -437,6 +445,9 @@ MTX.prototype.signInput = function signInput(index, addr, key, type) {
     if (!utils.equal(addr.keyHash, prev.get(2)))
       return false;
 
+    if (!Script.isKey(vector.get(1)))
+      throw new Error('Input has not been templated.');
+
     vector.set(0, signature);
     vector.compile();
 
@@ -476,6 +487,9 @@ MTX.prototype.signInput = function signInput(index, addr, key, type) {
     n = keys.length;
   }
 
+  if (vector.getSmall(0) !== 0)
+    throw new Error('Input has not been templated.');
+
   // Something is very wrong here. Abort.
   if (len - 1 > n)
     return false;
@@ -582,6 +596,8 @@ MTX.prototype.isSigned = function isSigned() {
     // signing p2sh transactions.
     if (prev.isScripthash()) {
       prev = input.script.getRedeem();
+      if (!prev)
+        return false;
       len = vector.length - 1;
     }
 
@@ -590,6 +606,8 @@ MTX.prototype.isSigned = function isSigned() {
     // and potentially alter the length.
     if (prev.isWitnessScripthash()) {
       prev = input.witness.getRedeem();
+      if (!prev)
+        return false;
       vector = input.witness;
       len = vector.length - 1;
     } else if (prev.isWitnessPubkeyhash()) {
@@ -895,9 +913,9 @@ MTX.prototype.maxSize = function maxSize(options, force) {
       // simply add more of the fee to the change
       // output.
       // m value
-      m = options.m || 15;
+      m = options.m || 3;
       // n value
-      n = options.n || 15;
+      n = options.n || 3;
       // OP_0
       size += 1;
       // OP_PUSHDATA0 [signature] ...
@@ -1025,7 +1043,7 @@ MTX.prototype.selectCoins = function selectCoins(coins, options) {
     }
   }
 
-  if (options.fee) {
+  if (options.fee != null) {
     fee = options.fee;
 
     // Transfer `total` funds maximum.
@@ -1042,7 +1060,9 @@ MTX.prototype.selectCoins = function selectCoins(coins, options) {
       address: options.changeAddress,
       // In case we don't have a change address,
       // use a fake p2pkh output to gauge size.
-      keyHash: constants.ZERO_HASH.slice(0, 20),
+      script: !options.changeAddress
+        ? Script.createPubkeyhash(HASH160)
+        : null,
       value: 0
     });
 
@@ -1154,7 +1174,8 @@ MTX.prototype.fill = function fill(coins, options) {
 
   // Will only happen in rare cases where
   // we're redeeming all non-standard coins.
-  assert(changeAddress, 'No change address available.');
+  if (!changeAddress)
+    throw new Error('No change address available.');
 
   // Add coins to transaction.
   for (i = 0; i < result.coins.length; i++)
@@ -1172,10 +1193,10 @@ MTX.prototype.fill = function fill(coins, options) {
     // Do nothing. Change is added to fee.
     this.outputs.pop();
     this.changeIndex = -1;
-    assert(this.getFee() === result.fee + result.change);
+    assert.equal(this.getFee(), result.fee + result.change);
   } else {
     this.changeIndex = this.outputs.length - 1;
-    assert(this.getFee() === result.fee);
+    assert.equal(this.getFee(), result.fee);
   }
 
   return result;