diff --git a/src/merkleblock.h b/src/merkleblock.h
--- a/src/merkleblock.h
+++ b/src/merkleblock.h
@@ -148,6 +148,9 @@
  * From the peer-node's perspective, the SPV client is a "filtered node".
  * See BIP37 for details:
  * https://github.com/bitcoin/bips/blob/master/bip-0037.mediawiki
+ *
+ * NOTE: The class assumes that the given CBlock has *at least* 1 transaction.
+ * If the CBlock has 0 txs, it will hit an assertion.
  */
 class CMerkleBlock {
 public:
diff --git a/src/merkleblock.cpp b/src/merkleblock.cpp
--- a/src/merkleblock.cpp
+++ b/src/merkleblock.cpp
@@ -54,6 +54,10 @@
 
 uint256 CPartialMerkleTree::CalcHash(int height, unsigned int pos,
                                      const std::vector<uint256> &vTxid) {
+    // we can never have zero txs in a merkle block, we always need the
+    // coinbase tx if we do not have this assert, we can hit a memory
+    // access violation when indexing into vTxid
+    assert(vTxid.size() != 0);
     if (height == 0) {
         // hash at height 0 is the txids themself.
         return vTxid[pos];