Getting GetPersonaResponseMessage with error on requesting GetPersona

I try to get a persona with GetPersona from EWS, but i'm only get an error. All other requests work without problems. This is my reqeust:

<soap:Body>
    <m:FindItem Traversal="Shallow">
      <m:ItemShape>
        <t:BaseShape>IdOnly</t:BaseShape>
        <t:AdditionalProperties>
          <t:FieldURI FieldURI="persona:DisplayName" />
        </t:AdditionalProperties>
      </m:ItemShape>
      <m:IndexedPageItemView MaxEntriesReturned="100" Offset="0" BasePoint="Beginning" />
      <m:ParentFolderIds>
        <t:DistinguishedFolderId Id="directory" />
      </m:ParentFolderIds>
    </m:FindItem>
  </soap:Body>

The response is:

<?xml version="1.0" encoding="utf-8"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
    <s:Header>
        <h:ServerVersionInfo MajorVersion="15" MinorVersion="20" MajorBuildNumber="7181" MinorBuildNumber="16" Version="V2018_01_08" xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" />
    </s:Header>
    <s:Body>
        <GetPersonaResponseMessage ResponseClass="Error" xmlns="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
            <MessageText>The request is invalid.</MessageText>
            <ResponseCode>ErrorInvalidOperation</ResponseCode>
            <DescriptiveLinkKey>0</DescriptiveLinkKey>
        </GetPersonaResponseMessage>
    </s:Body>
</s:Envelope>

I'm a little bit confused, that I'm getting a GetPersonaResponseMessage, but with an error in it. The example looks exactly the same: https://github.com/MicrosoftDocs/office-developer-exchange-docs/blob/main/docs/web-service-reference/getpersona-operation.md I'm a little bit stuck. Can somebody help me? Thanks!