diff --git a/ps.go b/ps.go
index 228f161a..b8a1b111 100644
--- a/ps.go
+++ b/ps.go
@@ -20,7 +20,7 @@ var psCommand = cli.Command{
 	Flags: []cli.Flag{
 		cli.StringFlag{
 			Name:  "format, f",
-			Value: "",
+			Value: "table",
 			Usage: `select one of: ` + formatOptions,
 		},
 	},
@@ -38,8 +38,12 @@ var psCommand = cli.Command{
 			return err
 		}
 
-		if context.String("format") == "json" {
+		switch context.String("format") {
+		case "table":
+		case "json":
 			return json.NewEncoder(os.Stdout).Encode(pids)
+		default:
+			return fmt.Errorf("invalid format option")
 		}
 
 		// [1:] is to remove command name, ex: