diff --git a/cmd/mxl-player/main.go b/cmd/mxl-player/main.go
index 6e5af7a..8f0fa77 100644
--- a/cmd/mxl-player/main.go
+++ b/cmd/mxl-player/main.go
@@ -243,7 +243,7 @@ func main() {
 
 	switch {
 	case args.VideoFlowId != "" && args.AudioFlowId != "":
-		syncSrc, err = source.OpenSync(args.Domain, args.VideoFlowId, args.AudioFlowId)
+		syncSrc, err = source.OpenSameDomainSync(args.Domain, args.VideoFlowId, args.AudioFlowId)
 		if err != nil {
 			log.Fatalf("sync source: %v", err)
 		}
@@ -378,7 +378,7 @@ func main() {
 		// Try once. Return error if fails — caller loops back to select
 		// and can pick up new reconnect params or a new grant.
 		if params.video != "" && params.audio != "" {
-			s, e := source.OpenSync(params.domain, params.video, params.audio)
+			s, e := source.OpenSameDomainSync(params.domain, params.video, params.audio)
 			if e == nil {
 				if r != nil {
 					newSize := vk.DeviceSize(s.Stride()) * vk.DeviceSize(s.Height())
diff --git a/internal/source/source.go b/internal/source/source.go
index 5f34696..e5186f0 100644
--- a/internal/source/source.go
+++ b/internal/source/source.go
@@ -305,7 +305,10 @@ type SyncSource struct {
 	width, height, stride uint32
 }
 
-func OpenSync(domain, videoFlow, audioFlow string) (*SyncSource, error) {
+// OpenSameDomainSync opens a native MXL synchronization group.
+// Both feeds must belong to the supplied domain because go-mxl sync groups
+// cannot contain readers from different MXL instances.
+func OpenSameDomainSync(domain, videoFlow, audioFlow string) (*SyncSource, error) {
 	inst, err := mxl.NewInstance(domain, "")
 	if err != nil {
 		return nil, fmt.Errorf("NewInstance: %w", err)