diff --git a/src/shrpx.cc b/src/shrpx.cc
index 15d11867..ed7ff18f 100644
--- a/src/shrpx.cc
+++ b/src/shrpx.cc
@@ -3479,9 +3479,12 @@ int process_options(Config *config,
     return -1;
   }
 
+  std::array<char, util::max_hostport> hostport_buf;
+
   auto &proxy = config->downstream_http_proxy;
   if (!proxy.host.empty()) {
-    auto hostport = util::make_hostport(StringRef{proxy.host}, proxy.port);
+    auto hostport = util::make_hostport(std::begin(hostport_buf),
+                                        StringRef{proxy.host}, proxy.port);
     if (resolve_hostname(&proxy.addr, proxy.host.c_str(), proxy.port,
                          AF_UNSPEC) == -1) {
       LOG(FATAL) << "Resolving backend HTTP proxy address failed: " << hostport;
@@ -3494,7 +3497,8 @@ int process_options(Config *config,
   {
     auto &memcachedconf = tlsconf.session_cache.memcached;
     if (!memcachedconf.host.empty()) {
-      auto hostport = util::make_hostport(StringRef{memcachedconf.host},
+      auto hostport = util::make_hostport(std::begin(hostport_buf),
+                                          StringRef{memcachedconf.host},
                                           memcachedconf.port);
       if (resolve_hostname(&memcachedconf.addr, memcachedconf.host.c_str(),
                            memcachedconf.port, memcachedconf.family) == -1) {
@@ -3515,7 +3519,8 @@ int process_options(Config *config,
   {
     auto &memcachedconf = tlsconf.ticket.memcached;
     if (!memcachedconf.host.empty()) {
-      auto hostport = util::make_hostport(StringRef{memcachedconf.host},
+      auto hostport = util::make_hostport(std::begin(hostport_buf),
+                                          StringRef{memcachedconf.host},
                                           memcachedconf.port);
       if (resolve_hostname(&memcachedconf.addr, memcachedconf.host.c_str(),
                            memcachedconf.port, memcachedconf.family) == -1) {
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index f07e48dd..a9886119 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -4332,6 +4332,8 @@ int configure_downstream_group(Config *config, bool http2_proxy,
 
   auto resolve_flags = numeric_addr_only ? AI_NUMERICHOST | AI_NUMERICSERV : 0;
 
+  std::array<char, util::max_hostport> hostport_buf;
+
   for (auto &g : addr_groups) {
     std::unordered_map<StringRef, uint32_t> wgchk;
     for (auto &addr : g.addrs) {
@@ -4377,7 +4379,7 @@ int configure_downstream_group(Config *config, bool http2_proxy,
           util::make_http_hostport(downstreamconf.balloc, addr.host, addr.port);
 
       auto hostport =
-          util::make_hostport(downstreamconf.balloc, addr.host, addr.port);
+          util::make_hostport(std::begin(hostport_buf), addr.host, addr.port);
 
       if (!addr.dns) {
         if (resolve_hostname(&addr.addr, addr.host.c_str(), addr.port,
diff --git a/src/util.cc b/src/util.cc
index 97af8d79..4491a07d 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -1329,18 +1329,6 @@ StringRef make_http_hostport(BlockAllocator &balloc, const StringRef &host,
   return make_http_hostport(iov.base, host, port);
 }
 
-std::string make_hostport(const StringRef &host, uint16_t port) {
-  std::string hostport;
-  // I'm not sure we can write \0 at the position std::string::size(),
-  // so allocate an extra byte.
-  hostport.resize(host.size() + 2 + 1 + 5 + 1);
-
-  auto s = make_hostport(std::begin(hostport), host, port);
-  hostport.resize(s.size());
-
-  return hostport;
-}
-
 StringRef make_hostport(BlockAllocator &balloc, const StringRef &host,
                         uint16_t port) {
   auto iov = make_byte_ref(balloc, host.size() + 2 + 1 + 5 + 1);
diff --git a/src/util.h b/src/util.h
index d4990e03..9c34d9e6 100644
--- a/src/util.h
+++ b/src/util.h
@@ -764,8 +764,6 @@ std::string format_duration(double t);
 
 // Just like make_http_hostport(), but doesn't treat 80 and 443
 // specially.
-std::string make_hostport(const StringRef &host, uint16_t port);
-
 StringRef make_hostport(BlockAllocator &balloc, const StringRef &host,
                         uint16_t port);
 
diff --git a/src/util_test.cc b/src/util_test.cc
index d5ff942f..17d640d5 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -540,22 +540,19 @@ void test_util_make_http_hostport(void) {
 }
 
 void test_util_make_hostport(void) {
+  std::array<char, util::max_hostport> hostport_buf;
   CU_ASSERT("localhost:80" ==
-            util::make_hostport(StringRef::from_lit("localhost"), 80));
-  CU_ASSERT("[::1]:443" ==
-            util::make_hostport(StringRef::from_lit("::1"), 443));
+            util::make_hostport(std::begin(hostport_buf),
+                                StringRef::from_lit("localhost"), 80));
+  CU_ASSERT("[::1]:443" == util::make_hostport(std::begin(hostport_buf),
+                                               StringRef::from_lit("::1"),
+                                               443));
 
   BlockAllocator balloc(4096, 4096);
   CU_ASSERT("localhost:80" ==
             util::make_hostport(balloc, StringRef::from_lit("localhost"), 80));
   CU_ASSERT("[::1]:443" ==
             util::make_hostport(balloc, StringRef::from_lit("::1"), 443));
-
-  // Check std::string version
-  CU_ASSERT(
-      "abcdefghijklmnopqrstuvwxyz0123456789:65535" ==
-      util::make_hostport(
-          StringRef::from_lit("abcdefghijklmnopqrstuvwxyz0123456789"), 65535));
 }
 
 void test_util_strifind(void) {