diff --git a/cmd/buildkitd/main.go b/cmd/buildkitd/main.go
index 9835f3088..8bafcfdfd 100644
--- a/cmd/buildkitd/main.go
+++ b/cmd/buildkitd/main.go
@@ -135,6 +135,9 @@ func main() {
 	app.Flags = append(app.Flags, appFlags...)
 
 	app.Action = func(c *cli.Context) error {
+		if os.Geteuid() != 0 {
+			return errors.New("rootless mode requires to be executed as the mapped root in a user namespace; you may use RootlessKit for setting up the namespace")
+		}
 		ctx, cancel := context.WithCancel(appcontext.Context())
 		defer cancel()