Whats wrong with this command? (PS Add-MailboxFolderPermission...)

Hi,

I am trying to add calendar permissions for multiple users with this command:

Get-mailbox -Filter {(CustomAttribute1 -eq "manager")} | Select-Object alias | ForEach-Object {Add-MailboxFolderPermission $_.alias:\Calendar -User someuser -AccessRights Editor}

I get error:

A positional parameter cannot be found that accepts argument ':\Calendar'

But when I run:

Get-mailbox -Filter {(CustomAttribute1 -eq "manager")} | Select-Object alias

It all works fine, I get i list of aliases with C.Attr1 "manager".

What is wrong with my piped ForEached-Object???