REST API Admin - Groups AddUserAsAdmin:"Parameter PrincipalType or Identifier is missing or invalid"

I am a TENANT admin and I am trying to add a group to all workspaces. Specifically, I want to use this:  https://learn.microsoft.com/en-us/rest/api/power-bi/admin/groups-add-user-as-admin

I have used the API to add my user id but I am not able to use on a group. It seems the API doesn't work when you use a group. 

Here is my code

$jsonRequestBody=@{
  identifier            = "xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx"
  principalType         = "Group"
  groupUserAccessRight  = "Admin"
}|ConvertTo-Json

Connect-PowerBIServiceAccount

$workspaceId= "xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx"
Write-Output "https://api.powerbi.com/v1.0/myorg/admin/groups/{$workspaceId}/users"
#Invoke-PowerBIRestMethod -Url "https://api.powerbi.com/v1.0/myorg/groups/{$workspaceId}/users" -Method Post -Headers $authHeader  -Body $jsonRequestBody -Verbose
$request=Invoke-PowerBIRestMethod -Url " https://api.powerbi.com/v1.0/myorg/admin/groups/$workspaceId/users" -Method Post -Verbose -Body $jsonRequestBody

Invoke-PowerBIRestMethod : One or more errors occurred.
At line:13 char:10
+ $request=Invoke-PowerBIRestMethod -Url "https://api.powerbi.com/v1.0/ ...
+          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : WriteError: (Microsoft.Power...werBIRestMethod:InvokePowerBIRestMethod) [Invoke-PowerBIRestMethod], AggregateException
    + FullyQualifiedErrorId : One or more errors occurred.,Microsoft.PowerBI.Commands.Profile.InvokePowerBIRestMethod

There response is 
VERBOSE: Request Uri: https://api.powerbi.com/v1.0/myorg/admin/groups/xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxx/users
VERBOSE: Status Code: BadRequest (400)
Invoke-PowerBIRestMethod : Encountered errors when invoking the command: {
  "code": "InvalidRequest",
  "message": "Parameter PrincipalType or Identifier is missing or invalid"

The same code works well when I modifiy the body to
$jsonRequestBody=@{
  emailAddress            = "[email protected]"
  principalType         = "User"
  groupUserAccessRight  = "Admin"
}|ConvertTo-Json