diff --git a/common/bytes.go b/common/bytes.go
index ba81bbe9..711720aa 100644
--- a/common/bytes.go
+++ b/common/bytes.go
@@ -51,3 +51,12 @@ func (bz HexBytes) Bytes() []byte {
 func (bz HexBytes) String() string {
 	return strings.ToUpper(hex.EncodeToString(bz))
 }
+
+func (bz HexBytes) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'p':
+		s.Write([]byte(fmt.Sprintf("%p", bz)))
+	default:
+		s.Write([]byte(fmt.Sprintf("%X", []byte(bz))))
+	}
+}
diff --git a/merkle/simple_proof.go b/merkle/simple_proof.go
index 7b8f82b4..c81ed674 100644
--- a/merkle/simple_proof.go
+++ b/merkle/simple_proof.go
@@ -43,9 +43,9 @@ func (sp *SimpleProof) StringIndented(indent string) string {
 
 // Use the leafHash and innerHashes to get the root merkle hash.
 // If the length of the innerHashes slice isn't exactly correct, the result is nil.
+// Recursive impl.
 func computeHashFromAunts(index int, total int, leafHash []byte, innerHashes [][]byte) []byte {
-	// Recursive impl.
-	if index >= total {
+	if index >= total || index < 0 || total <= 0 {
 		return nil
 	}
 	switch total {
@@ -80,7 +80,7 @@ func computeHashFromAunts(index int, total int, leafHash []byte, innerHashes [][
 // The node and the tree is thrown away afterwards.
 // Exactly one of node.Left and node.Right is nil, unless node is the root, in which case both are nil.
 // node.Parent.Hash = hash(node.Hash, node.Right.Hash) or
-// 									  hash(node.Left.Hash, node.Hash), depending on whether node is a left/right child.
+// hash(node.Left.Hash, node.Hash), depending on whether node is a left/right child.
 type SimpleProofNode struct {
 	Hash   []byte
 	Parent *SimpleProofNode