diff --git a/lib/bcoin/tx.js b/lib/bcoin/tx.js
index c6c2a4f2..05d3cd10 100644
--- a/lib/bcoin/tx.js
+++ b/lib/bcoin/tx.js
@@ -198,7 +198,7 @@ TX.prototype.signatureHashV0 = function signatureHashV0(index, prev, type) {
   if (typeof type === 'string')
     type = constants.hashType[type];
 
-  assert(index >= 0 && index < copy.inputs.length);
+  assert(index >= 0 && index < this.inputs.length);
   assert(prev instanceof bcoin.script);
 
   // Clone the transaction.