diff --git a/src/transaction.js b/src/transaction.js
index 0926b84..80393b9 100644
--- a/src/transaction.js
+++ b/src/transaction.js
@@ -84,9 +84,11 @@ Transaction.prototype.addOutput = function(scriptPubKey, value) {
     scriptPubKey = address.toOutputScript()
   }
 
+  assert(scriptPubKey instanceof Script, 'Expected Address or Script, got ' + scriptPubKey)
+
   return (this.outs.push({
     script: scriptPubKey,
-    value: value,
+    value: value
   }) - 1)
 }