diff --git a/lib/primitives/block.js b/lib/primitives/block.js
index cb2a441f..9d9e4fda 100644
--- a/lib/primitives/block.js
+++ b/lib/primitives/block.js
@@ -932,7 +932,7 @@ Block.prototype.getWitnessSizes = function getWitnessSizes() {
 
 Block.isBlock = function isBlock(obj) {
   return obj
-    && obj.merkleRoot !== undefined
+    && typeof obj.merkleRoot === 'string'
     && typeof obj.getClaimed === 'function';
 };