diff --git a/libs/chapter12/reverse_string.cpp b/libs/chapter12/reverse_string.cpp
index c6dc7e1..aec0f1f 100644
--- a/libs/chapter12/reverse_string.cpp
+++ b/libs/chapter12/reverse_string.cpp
@@ -9,7 +9,7 @@
 
 void reverse_str(char *str){
   size_t len = strlen(str);
-  char *tmp = reinterpret_cast<char*>(malloc(len));
+  char *tmp = reinterpret_cast<char*>(malloc(len + 1));
   strcpy(tmp, str);
   for(unsigned int offset = 0; offset < len; ++offset){
     str[offset] = tmp[len-offset-1];