diff --git a/lib/launcher/launcher.go b/lib/launcher/launcher.go
index b39b7616..8298b7b9 100644
--- a/lib/launcher/launcher.go
+++ b/lib/launcher/launcher.go
@@ -158,6 +158,9 @@ func (l *Launcher) Context(ctx context.Context) *Launcher {
 
 // Set a command line argument to launch the browser.
 func (l *Launcher) Set(name flags.Flag, values ...string) *Launcher {
+	if strings.Contains(string(name), "=") {
+		panic("flag name should not contain '='")
+	}
 	l.Flags[l.normalizeFlag(name)] = values
 	return l
 }
diff --git a/lib/launcher/launcher_test.go b/lib/launcher/launcher_test.go
index 937e625a..f6f2854e 100644
--- a/lib/launcher/launcher_test.go
+++ b/lib/launcher/launcher_test.go
@@ -121,6 +121,10 @@ func TestLaunch(t *testing.T) {
 		_, err = launcher.NewManaged("ws://not-exists")
 		g.Err(err)
 	}
+
+	{
+		g.Panic(func() { launcher.New().Set("a=b") })
+	}
 }
 
 func TestLaunchUserMode(t *testing.T) {