PTC si viewprojecthistory cli vs. webservice

The command „si viewprojecthistory -P xxx/project.pj --fields=date,labels” works just fine with the cli, but if use the webservice

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:gen="http://webservice.mks.com/2009/Generic" xmlns:sch="http://webservice.mks.com/2009/Generic/schema">

 <soapenv:Header/>

 <soapenv:Body>

 <gen:command>

 <!--Optional:-->

 <arg0 TransactionId="x">

 <sch:Username>x</sch:Username>

 <sch:Password>x</sch:Password>

 <sch:Application>si</sch:Application>

 <sch:Command>viewprojecthistory</sch:Command>

 <!--Zero or more repetitions:-->

 <sch:Option Name="project">xxx/project.pj</sch:Option>

 <sch:Option Name="fields">date,labels</sch:Option>

 </arg0>

 </gen:command>

 </soapenv:Body>

</soapenv:Envelope>

i get followig Response

<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
 <env:Header/>
 <env:Body>
 <env:Fault>
 <faultcode>env:Server</faultcode>
 <faultstring>MKS170064: The command succeeded but there was an error processing the output in the web services layer.</faultstring>
 <detail>
 <ns1:MKSException class="com.mks.api.response.APIException" implication="MKS170064: The command succeeded but there was an error processing the output in the web services layer." xmlns:ns1="http://webservice.mks.com/2009/Generic/fault">Invalid valuelist type: "null".</ns1:MKSException>
 </detail>
 </env:Fault>
 </env:Body>
</env:Envelope>