diff --git a/lib/bcoin/block.js b/lib/bcoin/block.js
index 97aaa4f6..5157dbb5 100644
--- a/lib/bcoin/block.js
+++ b/lib/bcoin/block.js
@@ -236,9 +236,7 @@ Block.prototype.hasWitness = function hasWitness() {
  */
 
 Block.prototype.addTX = function addTX(tx) {
-  var index;
-  assert(!tx.mutable, 'Cannot add mutable TX to block.');
-  index = this.txs.push(tx) - 1;
+  var index = this.txs.push(tx) - 1;
   tx.setBlock(this, index);
   return index;
 };