Edit Firewall Rule with API

Hey Guys,

i need to change a Firewall Rule with the API.

I tried to many times and read the documentation, but i dont find the solution.

my XML:<Request APIVersion="1800.1">

<Login>

<Username>username</Username>

<Password passwordform="plain">password</Password>

</Login>
<Set operation="update">
<PolicyType>
<Name>test</Name>
<Description>test</Description>
<Status>Disable</Status>
<IPFamily>IPv4</IPFamily>
<Position>After</Position>
<After>
<Action>Drop</Action>
</After>
<Before>
<Action>Accept</Action>
</Before>
</PolicyType>
</Set>


</Request>

The Output is: 

<?xml version="1.0" encoding="UTF-8"?>\n<Response APIVersion="1800.1" IPS_CAT_VER="1">\n <Login>\n <status>Authentication Successful</status>\n </Login>\n <FirewallRule transactionid="">\n <Status code="501">Configuration parameters validation failed.</Status>\n <InvalidParams>\n <Params>/FirewallRule/PolicyType</Params>\n </InvalidParams>\n </FirewallRule>\n</Response>\n'

Can you help me?

Greetings

Jannik