diff --git a/internal/model/model.go b/internal/model/model.go
index ab23b5bf4..2414e47a4 100644
--- a/internal/model/model.go
+++ b/internal/model/model.go
@@ -424,6 +424,11 @@ func (m *Model) NeedFolderFiles(folder string, max int) ([]db.FileInfoTruncated,
 // Index is called when a new device is connected and we receive their full index.
 // Implements the protocol.Model interface.
 func (m *Model) Index(deviceID protocol.DeviceID, folder string, fs []protocol.FileInfo, flags uint32, options []protocol.Option) {
+	if flags != 0 {
+		l.Warnln("protocol error: unknown flags 0x%x in Index message", flags)
+		return
+	}
+
 	if debug {
 		l.Debugf("IDX(in): %s %q: %d files", deviceID, folder, len(fs))
 	}
@@ -476,6 +481,11 @@ func (m *Model) Index(deviceID protocol.DeviceID, folder string, fs []protocol.F
 // IndexUpdate is called for incremental updates to connected devices' indexes.
 // Implements the protocol.Model interface.
 func (m *Model) IndexUpdate(deviceID protocol.DeviceID, folder string, fs []protocol.FileInfo, flags uint32, options []protocol.Option) {
+	if flags != 0 {
+		l.Warnln("protocol error: unknown flags 0x%x in IndexUpdate message", flags)
+		return
+	}
+
 	if debug {
 		l.Debugf("%v IDXUP(in): %s / %q: %d files", m, deviceID, folder, len(fs))
 	}
@@ -682,6 +692,11 @@ func (m *Model) Request(deviceID protocol.DeviceID, folder, name string, offset
 		return nil, ErrNoSuchFile
 	}
 
+	if flags != 0 {
+		// We don't currently support or expect any flags.
+		return nil, fmt.Errorf("protocol error: unknown flags 0x%x in Request message", flags)
+	}
+
 	// Verify that the requested file exists in the local model.
 	m.fmut.RLock()
 	folderFiles, ok := m.folderFiles[folder]