Encountered unmappable character error in powershell

Hi,

I am using a following cmdlets to get all the machines information from AD of our domain. After running this script i get only few machine around (10 % of total machine in AD).

Error - An error pops up "  Encountered unmappable character error " .

Que - What does these mean? And what more switches i need to add to get all machines ?  

Cmdlet used =  Get-ADComputer -Filter * -Property * | Select-Object Name,OperatingSystem | Export-CSV AllWindows.csv -                        NoTypeInformation -Encoding UTF8

Powershell Version used = 2.0

please let me know if more information is requird.