Error while removing access

I am trying to remove access for a group from an OU, however I am getting error:

Cannot convert argument "rule" with value:"System.Object[]", for RemoveAccessRule to type "System.DirectoryServices.ActiveDirectoryAccessRule

There is an error on line 4.

I am running the below:

$ou='OU=testou,DC=lab,dc=local'
$a= Get-ACL ("AD:\"+($ou))
$r=$a.Access | Where { $_.IdentityReference -eq 'lab\group1'}
$a.RemoveAccessRule($r)
$a | Set-ACL