diff --git a/src/shrpx.cc b/src/shrpx.cc
index 3c7dd888..bd250316 100644
--- a/src/shrpx.cc
+++ b/src/shrpx.cc
@@ -574,7 +574,7 @@ int create_unix_domain_server_socket(UpstreamAddr &faddr,
                 << (faddr.tls ? ", tls" : "");
     (*found).used = true;
     faddr.fd = (*found).fd;
-    faddr.hostport = "localhost";
+    faddr.hostport = ImmutableString::from_lit("localhost");
 
     return 0;
   }
@@ -639,7 +639,7 @@ int create_unix_domain_server_socket(UpstreamAddr &faddr,
               << (faddr.tls ? ", tls" : "");
 
   faddr.fd = fd;
-  faddr.hostport = "localhost";
+  faddr.hostport = ImmutableString::from_lit("localhost");
 
   return 0;
 }
@@ -855,7 +855,7 @@ get_inherited_addr_from_config(const Config *config) {
       continue;
     }
 
-    iaddr.host = host.data();
+    iaddr.host = ImmutableString{host.data()};
   }
 
   return iaddrs;
@@ -947,7 +947,7 @@ std::vector<InheritedAddr> get_inherited_addr_from_env() {
       }
 
       InheritedAddr addr{};
-      addr.host = path;
+      addr.host = ImmutableString{path};
       addr.host_unix = true;
       addr.fd = static_cast<int>(fd);
       iaddrs.push_back(std::move(addr));
@@ -1001,7 +1001,7 @@ std::vector<InheritedAddr> get_inherited_addr_from_env() {
       }
 
       InheritedAddr addr{};
-      addr.host = host.data();
+      addr.host = ImmutableString{host.data()};
       addr.port = static_cast<uint16_t>(port);
       addr.fd = static_cast<int>(fd);
       iaddrs.push_back(std::move(addr));
@@ -1273,7 +1273,7 @@ constexpr auto DEFAULT_ACCESSLOG_FORMAT = StringRef::from_lit(
 namespace {
 void fill_default_config(Config *config) {
   config->num_worker = 1;
-  config->conf_path = "/etc/nghttpx/nghttpx.conf";
+  config->conf_path = ImmutableString::from_lit("/etc/nghttpx/nghttpx.conf");
   config->pid = getpid();
 
   if (ev_supported_backends() & ~ev_recommended_backends() & EVBACKEND_KQUEUE) {
@@ -1304,7 +1304,8 @@ void fill_default_config(Config *config) {
     auto &ocspconf = tlsconf.ocsp;
     // ocsp update interval = 14400 secs = 4 hours, borrowed from h2o
     ocspconf.update_interval = 4_h;
-    ocspconf.fetch_ocsp_response_file = PKGDATADIR "/fetch-ocsp-response";
+    ocspconf.fetch_ocsp_response_file =
+        ImmutableString::from_lit(PKGDATADIR "/fetch-ocsp-response");
   }
 
   {
@@ -1375,7 +1376,7 @@ void fill_default_config(Config *config) {
     accessconf.format = parse_log_format(DEFAULT_ACCESSLOG_FORMAT);
 
     auto &errorconf = loggingconf.error;
-    errorconf.file = "/dev/stderr";
+    errorconf.file = ImmutableString::from_lit("/dev/stderr");
   }
 
   loggingconf.syslog_facility = LOG_DAEMON;
@@ -2397,7 +2398,7 @@ int process_options(Config *config,
 
   if (listenerconf.addrs.empty()) {
     UpstreamAddr addr{};
-    addr.host = "*";
+    addr.host = ImmutableString::from_lit("*");
     addr.port = 3000;
     addr.tls = true;
     addr.family = AF_INET;
@@ -2937,7 +2938,7 @@ int main(int argc, char **argv) {
         break;
       case 12:
         // --conf
-        mod_config()->conf_path = optarg;
+        mod_config()->conf_path = ImmutableString{optarg};
         break;
       case 14:
         // --syslog-facility
diff --git a/src/shrpx_config.cc b/src/shrpx_config.cc
index 08c9fa8a..91797dfb 100644
--- a/src/shrpx_config.cc
+++ b/src/shrpx_config.cc
@@ -2032,7 +2032,7 @@ int parse_config(Config *config, int optid, const StringRef &opt,
                  << strerror(errno);
       return -1;
     }
-    config->user = pwd->pw_name;
+    config->user = ImmutableString{pwd->pw_name};
     config->uid = pwd->pw_uid;
     config->gid = pwd->pw_gid;
 
@@ -2484,14 +2484,14 @@ int parse_config(Config *config, int optid, const StringRef &opt,
     switch (optid) {
     case SHRPX_OPTID_TLS_SESSION_CACHE_MEMCACHED: {
       auto &memcachedconf = config->tls.session_cache.memcached;
-      memcachedconf.host = host;
+      memcachedconf.host = ImmutableString{host};
       memcachedconf.port = port;
       memcachedconf.tls = params.tls;
       break;
     }
     case SHRPX_OPTID_TLS_TICKET_KEY_MEMCACHED: {
       auto &memcachedconf = config->tls.ticket.memcached;
-      memcachedconf.host = host;
+      memcachedconf.host = ImmutableString{host};
       memcachedconf.port = port;
       memcachedconf.tls = params.tls;
       break;
@@ -3005,7 +3005,7 @@ int configure_downstream_group(Config *config, bool http2_proxy,
         // for AF_UNIX socket, we use "localhost" as host for backend
         // hostport.  This is used as Host header field to backend and
         // not going to be passed to any syscalls.
-        addr.hostport = "localhost";
+        addr.hostport = ImmutableString::from_lit("localhost");
 
         auto path = addr.host.c_str();
         auto pathlen = addr.host.size();
diff --git a/src/template.h b/src/template.h
index f7e8cd86..496ac178 100644
--- a/src/template.h
+++ b/src/template.h
@@ -255,7 +255,8 @@ public:
   ImmutableString() : len(0), base("") {}
   ImmutableString(const char *s, size_t slen)
       : len(slen), base(copystr(s, s + len)) {}
-  ImmutableString(const char *s) : len(strlen(s)), base(copystr(s, s + len)) {}
+  explicit ImmutableString(const char *s)
+      : len(strlen(s)), base(copystr(s, s + len)) {}
   ImmutableString(const std::string &s)
       : len(s.size()), base(copystr(std::begin(s), std::end(s))) {}
   template <typename InputIt>
diff --git a/src/template_test.cc b/src/template_test.cc
index 9c77451b..4a773158 100644
--- a/src/template_test.cc
+++ b/src/template_test.cc
@@ -156,7 +156,7 @@ void test_template_string_ref(void) {
   CU_ASSERT(5 == from_lit.size());
 
   // from ImmutableString
-  ImmutableString im = "bravo";
+  auto im = ImmutableString::from_lit("bravo");
 
   StringRef imref(im);