diff --git a/lib/primitives/outpoint.js b/lib/primitives/outpoint.js
index db07be5e..c0013035 100644
--- a/lib/primitives/outpoint.js
+++ b/lib/primitives/outpoint.js
@@ -245,6 +245,7 @@ Outpoint.fromJSON = function fromJSON(json) {
 Outpoint.prototype.fromTX = function fromTX(tx, index) {
   assert(tx);
   assert(typeof index === 'number');
+  assert(index >= 0);
   this.hash = tx.hash('hex');
   this.index = index;
   return this;