"Messages": ["Key word \"attachments\" is missing."], "ReturnCode": 71 When updating via REST API

I'm attempting to update Service Manager request task fields via the following URL to a REST API and receive the response below from the web server.  Any ideas on how to troubleshoot or fix this would be appreciated.

{
  "Messages": ["Key word \"attachments\" is missing."],
  "ReturnCode": 71
}

An example URL (server and user name redacted for security purposes):

http://<servername>:13087/SM/9/rest/xcelrequesttasks/RFT00799645/action/Update System.Net.HttpWebRequest { "RequestTask": { "xcelDataCollectYesNo":"No", "ClosureCode":1, "ClosureComments":"Add user CN=<User Name>,OU=Minneapolis Service Region,OU=TCUsers,DC=corp,DC=xcelenergy,DC=net to or from group TCORP us-usr-inf-adm Adding user <domain username> to group TCORP us-usr-inf-adm was successful" }}

The section of the Powershell script calling this:

######################################
#Function to create the JSON object
######################################
function AddJsonToRequest {
 param($rq, $body);
 $bodyStream = $rq.GetRequestStream();
 $bodyWriter = New-Object System.IO.StreamWriter($bodyStream);
 $bodyWriter.Write($body);
 $bodyWriter.Close();
 $bodyStream.Close();

########################################
#Writing back to the HPSM Task record
########################################

$cred = "<username>:<password>";
$cred = [System.Text.Encoding]::GetEncoding("ISO-8859-1").GetBytes($cred);
$cred = [System.Convert]::ToBase64String($cred);
$url = ("{0}/SM/9/rest/xcelrequesttasks/{1}/action/Update" -f $baseHpsmUrl, $taskId);
$rq = [System.Net.WebRequest]::Create($url);
$rq.Method = "POST";
$rq.ContentType = "application/json";
$rq.Timeout = 100000;
$rq.ReadWriteTimeout = 100000;
$rq.Headers.Add("Authorization", "Basic " $cred);

if($finalStatus -eq "ok") {
 $xcelDataCollectYesNo="No";
 $ClosureCode = '1';
} else {
 $xcelDataCollectYesNo="Yes";
 $ClosureCode = '3';
}

$ClosureComments = [string]::Join(" ", $comments);
$ClosureComments = $ClosureComments -replace '"', ""
$ClosureComments = $ClosureComments -replace '\\', ""

$body = '{{ "RequestTask": {{ "xcelDataCollectYesNo":"{0}", "ClosureCode":{1}, "ClosureComments":"{2}" }}}} ';
$body = ($body -f $xcelDataCollectYesNo, $ClosureCode, $ClosureComments );

AddJsonToRequest $rq $body
$rq.GetResponse();