diff --git a/lib/bcoin/mtx.js b/lib/bcoin/mtx.js
index 2015be97..c7eef3e6 100644
--- a/lib/bcoin/mtx.js
+++ b/lib/bcoin/mtx.js
@@ -127,10 +127,12 @@ MTX.prototype.addInput = function addInput(options, index) {
 
   assert(this.ts === 0, 'Cannot modify a confirmed tx.');
 
-  if (options instanceof MTX)
+  if (options instanceof bcoin.tx)
     options = bcoin.coin(options, index);
 
   if (options instanceof bcoin.coin) {
+    assert(typeof options.hash === 'string');
+    assert(typeof options.index === 'number');
     options = {
       prevout: { hash: options.hash, index: options.index },
       coin: options
@@ -901,7 +903,7 @@ MTX.prototype.selectCoins = function selectCoins(coins, options) {
     var i, index;
 
     for (i = lastAdded; i < coins.length; i++) {
-      // Add new inputs until MTX will have enough
+      // Add new inputs until TX will have enough
       // funds to cover both minimum post cost
       // and fee.
       tx.addInput(coins[i]);
@@ -929,7 +931,7 @@ MTX.prototype.selectCoins = function selectCoins(coins, options) {
     addCoins();
 
     // Add dummy output (for `change`) to
-    // calculate maximum MTX size.
+    // calculate maximum TX size.
     tx.addOutput({
       address: options.changeAddress,
       value: new bn(0)
diff --git a/lib/bcoin/tx.js b/lib/bcoin/tx.js
index 3e5bedb7..1b1ae060 100644
--- a/lib/bcoin/tx.js
+++ b/lib/bcoin/tx.js
@@ -601,9 +601,12 @@ TX.prototype.fillCoins = function fillCoins(coins) {
     coins = coins.reduce(function(out, coin) {
       if (coin instanceof TX) {
         out[coin.hash('hex')] = coin;
-      } else {
+      } else if (coin instanceof bcoin.coin) {
         assert(typeof coin.hash === 'string');
+        assert(typeof coin.index === 'number');
         out[coin.hash + '/' + coin.index] = coin;
+      } else {
+        assert(false, 'Non-coin object passed to fillCoins.');
       }
       return out;
     }, {});