diff --git a/ch2/ourown/fenwicktree_ds.cpp b/ch2/ourown/fenwicktree_ds.cpp
index becf380..56777a3 100644
--- a/ch2/ourown/fenwicktree_ds.cpp
+++ b/ch2/ourown/fenwicktree_ds.cpp
@@ -52,7 +52,7 @@ class FenwickTree {                              // index 0 is not used
     while (p*2 < (int)ft.size()) p *= 2;
     int i = 0;
     while (p) {
-      if (k > ft[i+p]) {
+      if (i+p < (int) ft.size() && k > ft[i+p]) {
         k -= ft[i+p];
         i += p;
       }
diff --git a/ch2/ourown/fenwicktree_ds.py b/ch2/ourown/fenwicktree_ds.py
index 2c58123..0a5fa26 100644
--- a/ch2/ourown/fenwicktree_ds.py
+++ b/ch2/ourown/fenwicktree_ds.py
@@ -33,7 +33,7 @@ def select(self, k):
 
         i = 0
         while p > 0:
-            if k > self.ft[i + p]:
+            if i + p < len(self.ft) and k > self.ft[i + p]:
                 k -= self.ft[i + p]
                 i += p
             p //= 2