Updating User Notification Settings via API

[rpayne4]

Hello, 
I'm generating a group of users via a SIS upload then trying to go through each profile and switch off all notifications for each user. Currently I have a script that looks up the user's communication channel id and then tries to switch just the first notification preference to "never". 

But on the second step, I'm hitting a 500 error and I'm not sure what I'm getting wrong. I'm sure it's something simple. Here's the segment of my code I'm having trouble with:

foreach($parent in $Parents){
   $apiURL1 = $domain+"/api/v1/users/sis_user_id:"+$parent.user_id+"/communication_channels/"
   $data1 = (Invoke-WebRequest -Headers $headers -Method GET -ContentType 'application/json' -Uri $apiURL1)
   $userChannel = $data1.Content | ConvertFrom-Json

   $channelAPI =    $domain+"/api/v1/users/self/communication_channels/"+$userChannel.id+"/notification_preferences?as_user_id=sis_user_id:"+$parent.user_id
   $BodyData = @{"notification_preferences[course_content][frequency]" = "never"; "notification_preferences[due_date][frequency]" = "never"}
   $json = $BodyData | ConvertTo-Json

   $data2 = (Invoke-WebRequest -Headers $headers -Method PUT -ContentType 'application/json' -Uri $channelAPI -Body $json)
}

Any ideas?

Thanks,
Rob