diff --git a/lib/bcoin/mtx.js b/lib/bcoin/mtx.js
index dec96216..1346f37c 100644
--- a/lib/bcoin/mtx.js
+++ b/lib/bcoin/mtx.js
@@ -803,13 +803,8 @@ MTX.prototype.maxSize = function maxSize(maxM, maxN) {
 };
 
 MTX.prototype.selectCoins = function selectCoins(coins, options) {
-  var tx = this.clone();
-  var outputValue = tx.getOutputValue();
-  var chosen = [];
-  var lastAdded = 0;
-  var tryFree, minValue, i, size, change, fee;
-
-  assert(tx.inputs.length === 0);
+  var chosen, lastAdded, tx, outputValue;
+  var i, size, change, fee, tryFree, minValue;
 
   if (!options || typeof options !== 'object') {
     options = {
@@ -818,6 +813,13 @@ MTX.prototype.selectCoins = function selectCoins(coins, options) {
     };
   }
 
+  tx = this.clone();
+  tx.inputs.length = 0;
+
+  chosen = [];
+  lastAdded = 0;
+
+  outputValue = tx.getOutputValue();
   tryFree = options.free;
 
   if (!options.selection || options.selection === 'age') {
@@ -957,6 +959,7 @@ MTX.prototype.fill = function fill(coins, options) {
   var result, err;
 
   assert(this.ts === 0, 'Cannot modify a confirmed tx.');
+  assert(this.inputs.length === 0, 'TX is already filled.');
 
   if (!options || typeof options !== 'object') {
     options = {