diff --git a/src/block.js b/src/block.js
index 23ae976..8e608e8 100644
--- a/src/block.js
+++ b/src/block.js
@@ -2,6 +2,8 @@ var bufferutils = require('./bufferutils')
 var bcrypto = require('./crypto')
 var bufferReverse = require('buffer-reverse')
 var fastMerkleRoot = require('merkle-lib/fastRoot')
+var typeforce = require('typeforce')
+var types = require('./types')
 
 var Transaction = require('./transaction')
 
@@ -134,6 +136,7 @@ Block.calculateTarget = function (bits) {
 }
 
 Block.calculateMerkleRoot = function (transactions) {
+  typeforce([{ getHash: types.Function }], transactions)
   if (transactions.length === 0) throw TypeError('Cannot compute merkle root for zero transactions')
 
   var hashes = transactions.map(function (transaction) {