diff --git a/compiler/back_end/cpp/testcode/auto_array_size_test.cc b/compiler/back_end/cpp/testcode/auto_array_size_test.cc
index eaae8ec..e3456e2 100644
--- a/compiler/back_end/cpp/testcode/auto_array_size_test.cc
+++ b/compiler/back_end/cpp/testcode/auto_array_size_test.cc
@@ -306,7 +306,7 @@ TEST(AutoSizeView, CanCopyFrom) {
   auto source = MakeAlignedAutoSizeView<const ::std::uint8_t, 8>(
       kAutoSize, sizeof kAutoSize);
 
-  ::std::array</**/ ::std::uint8_t, sizeof kAutoSize> buf = {0};
+  alignas(8) ::std::array</**/ ::std::uint8_t, sizeof kAutoSize> buf = {0};
   auto dest =
       MakeAlignedAutoSizeView</**/ ::std::uint8_t, 8>(buf.data(), buf.size());