Using a variable for $db.SetOwner

Does anyone know how I can use a variable for this?

$owner = "someowner"
$srv = new-Object Microsoft.SqlServer.Management.Smo.Server($destinationInstance)
$db = New-Object Microsoft.SqlServer.Management.Smo.Database
$db = $srv.Databases.Item($sourceDbName)
$db.SetOwner($owner, $TRUE)

Gives me errors when I try the above.

If I use $db.SetOwner('sa', $true) is works but I need to hold a variable.