klien zeep SOAP pengkodean double base64 yang tidak terduga

Tolong bantu saya memahami mengapa nilai dalam permintaan saya dikodekan ke base64 dua kali:

bagian relevan dari wsdl, yang saya gunakan:

<wsdl:types>
      <xsd:schema targetNamespace="http://www.blabla.com/">
         <xsd:element name="createHeaderSecurityRequest" type="xsd:base64Binary"/>

 <wsdl:message name="createHeaderSecurityRequestMessage">
      <wsdl:part element="tns:createHeaderSecurityRequest" name="parameters"/>
   </wsdl:message>

<wsdl:operation name="createHeaderSecurity">
         <wsdl:input message="tns:createHeaderSecurityRequestMessage"/>
         <wsdl:output message="tns:createHeaderSecurityResponseMessage"/>
         <wsdl:fault message="tns:faultMessage" name="createHeaderSecurityFaultMessage"/>
/wsdl:operation>

kode saya:

from zeep import Client
message = 'my message, that needs to be encoded in base64 only once'
wsdl = 'http://xxxxxxx?wsdl'
client = Client(wsdl)
request = client.create_message(client.service, 'createHeaderSecurity', message.encode())

apa yang saya dapatkan (pesan secara otomatis dikodekan ke base64 dua kali): permintaan =

<soap-env:Body xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/">
   <ns0:createHeaderSecurityRequest xmlns:ns0="http://www.blabla.com/">YlhrZ2JXVnpjMkZuWlN3Z2RHaGhkQ0J1WldWa2N5QjBieUJpWlNCbGJtTnZaR1ZrSUdsdUlHSmhjMlUyTkNCdmJteDVJRzl1WTJVPQ==</ns0:createHeaderSecurityRequest>
</soap-env:Body>

apa yang saya inginkan (nilai pesan dikodekan ke base64 hanya sekali): permintaan =

<soap-env:Body xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/">
   <ns0:createHeaderSecurityRequest xmlns:ns0="http://www.blabla.com/">bXkgbWVzc2FnZSwgdGhhdCBuZWVkcyB0byBiZSBlbmNvZGVkIGluIGJhc2U2NCBvbmx5IG9uY2U=</ns0:createHeaderSecurityRequest>
</soap-env:Body>

Saya akan sangat berterima kasih atas bantuan apa pun

zeep 3.4.0 python 3.7.3