Display error instead of just say command is incorrect

app_test.go

@@ -252,6 +252,23 @@ func TestApp_RunAsSubcommandParseFlags(t *testing.T) {
 	expect(t, context.String("lang"), "spanish")
 }
 
+func TestApp_RunAsSubCommandIncorrectUsage(t *testing.T) {
+	a := App{
+		Flags: []Flag{
+			StringFlag{Name: "--foo"},
+		},
+		Writer: bytes.NewBufferString(""),
+	}
+
+	set := flag.NewFlagSet("", flag.ContinueOnError)
+	set.Parse([]string{"", "---foo"})
+	c := &Context{flagSet: set}
+
+	err := a.RunAsSubcommand(c)
+
+	expect(t, err, errors.New("bad flag syntax: ---foo"))
+}
+
 func TestApp_CommandWithFlagBeforeTerminator(t *testing.T) {
 	var parsedOption string
 	var args []string