diff --git a/src/transaction_builder.js b/src/transaction_builder.js
index d05ee5b..ddddb1a 100644
--- a/src/transaction_builder.js
+++ b/src/transaction_builder.js
@@ -82,7 +82,7 @@ function extractSignature(txIn) {
 }
 
 function TransactionBuilder() {
-  this.prevOutMap = {}
+  this.prevTxMap = {}
   this.prevOutScripts = {}
   this.prevOutTypes = {}
 
@@ -151,10 +151,10 @@ TransactionBuilder.prototype.addInput = function(prevTx, index, sequence, prevOu
   }), 'No, this would invalidate signatures')
 
   var prevOut = prevOutHash.toString('hex') + ':' + index
-  assert(!(prevOut in this.prevOutMap), 'Transaction is already an input')
+  assert(!(prevOut in this.prevTxMap), 'Transaction is already an input')
 
   var vout = this.tx.addInput(prevOutHash, index, sequence)
-  this.prevOutMap[prevOut] = true
+  this.prevTxMap[prevOut] = true
   this.prevOutScripts[vout] = prevOutScript
   this.prevOutTypes[vout] = prevOutType