diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -1,4 +1,4 @@
-import collections
+import collections
 import socket
 import ssl
 import logging as log
@@ -79,7 +79,8 @@ class Client:
 			for (j,i) in enumerate(indices):
 				(j1,j2)=[HashTree.HASH_LEN*ji for ji in (j,j+1)]
 				if localTree.store[i]!=binData[j1:j2]:
-					if 2*i+3<len(localTree.store): # inner node
+					# ie. 0-6 nodes, 7-14 leaves. 2*6+2<15
+					if 2*i+2<len(localTree.store): # inner node
 						frontier.append(2*i+1)
 						frontier.append(2*i+2)
 					else: