diff --git a/cmd/stdiscosrv/apisrv.go b/cmd/stdiscosrv/apisrv.go
index 5ac4622a1..a848ebc93 100644
--- a/cmd/stdiscosrv/apisrv.go
+++ b/cmd/stdiscosrv/apisrv.go
@@ -18,6 +18,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -279,6 +280,10 @@ func (s *apiSrv) handleAnnounce(remote net.IP, deviceID protocol.DeviceID, addre
 		dbAddrs[i].Expires = expire
 	}
 
+	// The address slice must always be sorted for database merges to work
+	// properly.
+	sort.Sort(databaseAddressOrder(dbAddrs))
+
 	seen := now.UnixNano()
 	if s.repl != nil {
 		s.repl.send(key, dbAddrs, seen)
diff --git a/cmd/stdiscosrv/database.go b/cmd/stdiscosrv/database.go
index d4d02536f..84a28b7ae 100644
--- a/cmd/stdiscosrv/database.go
+++ b/cmd/stdiscosrv/database.go
@@ -10,6 +10,7 @@
 package main
 
 import (
+	"log"
 	"sort"
 	"time"
 
@@ -263,12 +264,15 @@ func (s *levelDBStore) Stop() {
 // chosen for any duplicates.
 func merge(a, b DatabaseRecord) DatabaseRecord {
 	// Both lists must be sorted for this to work.
-	sort.Slice(a.Addresses, func(i, j int) bool {
-		return a.Addresses[i].Address < a.Addresses[j].Address
-	})
-	sort.Slice(b.Addresses, func(i, j int) bool {
-		return b.Addresses[i].Address < b.Addresses[j].Address
-	})
+	if !sort.IsSorted(databaseAddressOrder(a.Addresses)) {
+		log.Println("Warning: bug: addresses not correctly sorted in merge")
+		a.Addresses = sortedAddressCopy(a.Addresses)
+	}
+	if !sort.IsSorted(databaseAddressOrder(b.Addresses)) {
+		// no warning because this is the side we read from disk and it may
+		// legitimately predate correct sorting.
+		b.Addresses = sortedAddressCopy(b.Addresses)
+	}
 
 	res := DatabaseRecord{
 		Addresses: make([]DatabaseAddress, 0, len(a.Addresses)+len(b.Addresses)),
@@ -352,3 +356,24 @@ func expire(addrs []DatabaseAddress, now int64) []DatabaseAddress {
 	}
 	return addrs
 }
+
+func sortedAddressCopy(addrs []DatabaseAddress) []DatabaseAddress {
+	sorted := make([]DatabaseAddress, len(addrs))
+	copy(sorted, addrs)
+	sort.Sort(databaseAddressOrder(sorted))
+	return sorted
+}
+
+type databaseAddressOrder []DatabaseAddress
+
+func (s databaseAddressOrder) Less(a, b int) bool {
+	return s[a].Address < s[b].Address
+}
+
+func (s databaseAddressOrder) Swap(a, b int) {
+	s[a], s[b] = s[b], s[a]
+}
+
+func (s databaseAddressOrder) Len() int {
+	return len(s)
+}