diff --git a/tests/stack_protector_test_helper.cpp b/tests/stack_protector_test_helper.cpp
index 3f15a12..2db4ef1 100644
--- a/tests/stack_protector_test_helper.cpp
+++ b/tests/stack_protector_test_helper.cpp
@@ -21,6 +21,6 @@
   // the line of defense *after* that.
   // Without volatile, the generic x86/x86-64 targets don't write to the stack.
   volatile char* p = buf;
-  int size = static_cast<int>(sizeof(buf) + 1);
+  int size = static_cast<int>(sizeof(buf) + sizeof(void*));
   while ((p - buf) < size) *p++ = '\0';
 }