Update B2C Custom Attribute String Collection via AD Graph API

Can I define a custom attribute whose data type is stringCollection and update it via the Azure AD Graph API? My experiment is failing:

The request body contains unexpected characters/content for the specified content type and encoding

Custom Attribute Definition

<ClaimType Id="extension_array_test">
    <DisplayName>Array Test</DisplayName>
    <DataType>stringCollection</DataType>
    <UserHelpText>Array Test in Token</UserHelpText>
</ClaimType>  

Using the Graph API Sample client

{
    "extension_[my-guid]_array_test": ["value 1", "value 2"]
}

Error Calling the Graph API:
{
  "odata.error": {
    "code": "Request_BadRequest",
    "message": {
      "lang": "en",
      "value": "The request body contains unexpected characters/content for the specified content type and encoding."
    }
  }
}