Problems consuming parameters by propertyName from ADObjects using Alias

Hi!

I have a parameter named "signature" which I want to be able to pipe data to from ADObjects (from the module ActiveDirectory). Adding an alias named "samaccountname" to the parameter "signature" should populate "signature" with the samaccountnames from the ADObjects. But that do not work. If I put an "select * " in between in the pipe, it do work. Why? 

Code Example:

Set-StrictMode -Version Latest

function Do-StuffToObject {
  [cmdletbinding()]
  Param(
    [Alias("samaccountname")]
    [Parameter(Position=0,ValueFromPipelinebyPropertyName=$True)]
    [string] $signature = $null
  )
  PROCESS {
    Write-Host "Signatur: $signature"
  }
}

$obj = Get-ADUser kalle

# The following line generates output: "Signature: "
$obj |  Do-StuffToObject

# following line generates output: "Signature: kalle"
$obj | Select * |  Do-StuffToObject