I found this great bit of code online that generates uptime and disk space for all computers on your domain. The problem is it's slow....VERY slow, like over 24 hours slow. So I decided instead of forcing the script to Get-ADComputer, I would just dump all computers into a text file and have it read that. And that's when the trouble begins. Here's the error:

Test-Connection : Cannot validate argument on parameter 'ComputerName'. The
argument is null or empty. Provide an argument that is not null or empty, and
then try the command again.
At C:\Scripts\QueryAD_HTML_Uptime_FreespaceReport.ps1:68 char:28
      if(Test-Connection -cn $s -Quiet -BufferSize 16 -Count 1)
                             ~~
  CategoryInfo          : InvalidData: (:) [Test-Connection],    ParameterBindingValidationException
  FullyQualifiedErrorId :   ParameterArgumentValidationError,Microsoft.Power
 Shell.Commands.TestConnectionCommand

Here's the relevant bit of code:

$servers = Get-Content "C:\Scripts\Computers.txt"

Function Get-UpTime
{ Param ([string[]]$servers)

$computers = @()

Foreach ($s in $servers) 
{ 
 if(Test-Connection -cn $s -Quiet -BufferSize 16 -Count 1)