detection method for bitlocker failed

Hello,

The followed code work without problem to return "On" if bitlocker is well present.

But when this is use as detection method it looks like not supported !!!

$BitlockerStatus = (Get-BitLockerVolume -MountPoint "c:").ProtectionStatus

if($BitlockerStatus -eq "On")
{
      return "installed"
}

somebody knows about