diff --git a/lib/primitives/memblock.js b/lib/primitives/memblock.js
index 0f3c217c..e594957a 100644
--- a/lib/primitives/memblock.js
+++ b/lib/primitives/memblock.js
@@ -213,7 +213,10 @@ MemBlock.prototype.toHeaders = function toHeaders() {
  */
 
 MemBlock.isMemBlock = function isMemBlock(obj) {
-  return obj && obj.memory && typeof obj.toBlock === 'function';
+  return obj
+    && typeof obj.toBlock === 'function'
+    && typeof obj.isMemory === 'function'
+    && obj.isMemory();
 };
 
 /*