diff --git a/dbms/src/Functions/tests/gtest_strings_simd_consistency.cpp b/dbms/src/Functions/tests/gtest_strings_simd_consistency.cpp
index 7f04f3ee68d..92fc79eaad2 100644
--- a/dbms/src/Functions/tests/gtest_strings_simd_consistency.cpp
+++ b/dbms/src/Functions/tests/gtest_strings_simd_consistency.cpp
@@ -296,9 +296,9 @@ TEST(StringsLowerUpperAscii, Random)
     std::random_device device;
     auto seed = device();
     std::cout << "seeded with: " << seed << std::endl;
-    std::vector<UInt8> data(limit);
-    std::vector<UInt8> res_new(limit);
-    std::vector<UInt8> res_old(limit);
+    std::vector<UInt8> data(limit + 1);
+    std::vector<UInt8> res_new(limit + 1, 0);
+    std::vector<UInt8> res_old(limit + 1, 0);
     std::default_random_engine eng(seed);
     std::uniform_int_distribution<UInt8> dist(
         'A',
@@ -307,6 +307,7 @@ TEST(StringsLowerUpperAscii, Random)
     {
         i = dist(eng);
     }
+    data.back() = 0;
     {
         {
             auto begin = high_resolution_clock::now();
@@ -332,9 +333,9 @@ TEST(StringsLowerUpperUtf8, Random)
     std::random_device device;
     auto seed = device();
     std::cout << "seeded with: " << seed << std::endl;
-    std::vector<UInt8> data(limit);
-    std::vector<UInt8> res_new(limit, 0);
-    std::vector<UInt8> res_old(limit, 0);
+    std::vector<UInt8> data(limit + 1);
+    std::vector<UInt8> res_new(limit + 1, 0);
+    std::vector<UInt8> res_old(limit + 1, 0);
     std::default_random_engine eng(seed);
     std::uniform_int_distribution<UInt8> dist(
         std::numeric_limits<UInt8>::min(),
@@ -343,6 +344,7 @@ TEST(StringsLowerUpperUtf8, Random)
     {
         i = dist(eng);
     }
+    data.back() = 0;
     {
         {
             auto begin = high_resolution_clock::now();