Why should i add state?

Hi, I get this error and i don't know why ??

Start state is missing. Add at least one state to the flow

File : main-flow.xml

<?xml version="1.0" encoding="UTF-8"?>
<flow xmlns="http://ift.tt/1jwD1YA"
xmlns:xsi="http://ift.tt/ra1lAU"
xsi:schemaLocation="http://ift.tt/1jwD1YA
    http://ift.tt/1Jk0LtK">

<view-state id="start" view="start.xhtml">

</view-state>

File pom.xml

 <dependency>
        <groupId>org.springframework.webflow</groupId>
        <artifactId>spring-webflow</artifactId>
        <version>2.3.1.RELEASE</version>
    </dependency>