diff --git a/cmd/strelaypoolsrv/main.go b/cmd/strelaypoolsrv/main.go
index 5ec000fa2..17b668e68 100644
--- a/cmd/strelaypoolsrv/main.go
+++ b/cmd/strelaypoolsrv/main.go
@@ -314,8 +314,11 @@ func handleRequest(w http.ResponseWriter, r *http.Request) {
 
 func handleGetRequest(rw http.ResponseWriter, r *http.Request) {
 	rw.Header().Set("Content-Type", "application/json; charset=utf-8")
+
 	mut.RLock()
-	relays := append(permanentRelays, knownRelays...)
+	relays := make([]*relay, len(permanentRelays)+len(knownRelays))
+	n := copy(relays, permanentRelays)
+	copy(relays[n:], knownRelays)
 	mut.RUnlock()
 
 	// Shuffle
diff --git a/cmd/strelaypoolsrv/main_test.go b/cmd/strelaypoolsrv/main_test.go
new file mode 100644
index 000000000..6b5732f12
--- /dev/null
+++ b/cmd/strelaypoolsrv/main_test.go
@@ -0,0 +1,67 @@
+// Copyright © 2020 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+)
+
+func init() {
+	for i := 0; i < 10; i++ {
+		u := fmt.Sprintf("permanent%d", i)
+		permanentRelays = append(permanentRelays, &relay{URL: u})
+	}
+
+	knownRelays = []*relay{
+		{URL: "known1"},
+		{URL: "known2"},
+		{URL: "known3"},
+	}
+
+	mut = new(sync.RWMutex)
+}
+
+// Regression test: handleGetRequest should not modify permanentRelays.
+func TestHandleGetRequest(t *testing.T) {
+	needcap := len(permanentRelays) + len(knownRelays)
+	if needcap > cap(permanentRelays) {
+		t.Fatalf("test setup failed: need cap(permanentRelays) >= %d, have %d",
+			needcap, cap(permanentRelays))
+	}
+
+	w := httptest.NewRecorder()
+	w.Body = new(bytes.Buffer)
+	handleGetRequest(w, httptest.NewRequest("GET", "/", nil))
+
+	result := make(map[string][]*relay)
+	err := json.NewDecoder(w.Body).Decode(&result)
+	if err != nil {
+		t.Fatalf("invalid JSON: %v", err)
+	}
+
+	relays := result["relays"]
+	expect, actual := len(knownRelays)+len(permanentRelays), len(relays)
+	if actual != expect {
+		t.Errorf("expected %d relays, got %d", expect, actual)
+	}
+
+	// Check for changes in permanentRelays.
+	for i, r := range permanentRelays {
+		switch {
+		case !strings.HasPrefix(r.URL, "permanent"):
+			t.Errorf("relay %q among permanent relays", r.URL)
+		case r.URL != fmt.Sprintf("permanent%d", i):
+			t.Error("order of permanent relays changed")
+		}
+	}
+}