diff --git a/src/shrpx_tls.cc b/src/shrpx_tls.cc
index 9aa789d0..24ba7f9c 100644
--- a/src/shrpx_tls.cc
+++ b/src/shrpx_tls.cc
@@ -2046,9 +2046,9 @@ StringRef get_x509_serial(BlockAllocator &balloc, X509 *x) {
     return StringRef{};
   }
 
-  std::array<uint8_t, 8> b;
+  std::array<uint8_t, 20> b;
   auto n = BN_bn2bin(bn, b.data());
-  assert(n == b.size());
+  assert(n <= 20);
 
   return util::format_hex(balloc, StringRef{std::begin(b), std::end(b)});
 #endif // !OPENSSL_1_1_API