diff --git a/tests/run_unit_tests_size.c b/tests/run_unit_tests_size.c
index 0d0a1de36..2af425241 100644
--- a/tests/run_unit_tests_size.c
+++ b/tests/run_unit_tests_size.c
@@ -76,7 +76,7 @@ static int router_id_tests(void)
     //
 
     memset(boundary_id, 'B', 254);
-    boundary_id[255] = 0;
+    boundary_id[254] = 0;
 
     qd_router_id_initialize("0", boundary_id);
     id = qd_router_id();