diff --git a/core/src/core/mod.rs b/core/src/core/mod.rs
index cffccadb2..1095c70aa 100644
--- a/core/src/core/mod.rs
+++ b/core/src/core/mod.rs
@@ -163,13 +163,15 @@ impl Hashed for HPair {
 	}
 }
 /// An iterator over hashes in a vector that pairs them to build a row in a
-/// Merkle tree. If the vector has an odd number of hashes, duplicates the last.
+/// Merkle tree. If the vector has an odd number of hashes, it appends a zero hash
+/// See https://bitcointalk.org/index.php?topic=102395.0 CVE-2012-2459 (block merkle calculation exploit)
+/// for the argument against duplication of last hash
 struct HPairIter(Vec<Hash>);
 impl Iterator for HPairIter {
 	type Item = HPair;
 
 	fn next(&mut self) -> Option<HPair> {
-		self.0.pop().map(|first| HPair(first, self.0.pop().unwrap_or(first)))
+		self.0.pop().map(|first| HPair(first, self.0.pop().unwrap_or(ZERO_HASH)))
 	}
 }
 /// A row in a Merkle tree. Can be built from a vector of hashes. Calculates