Error when running CreateDefaultAssociatedGroups in powershell

I created a webapp and sitecollection from scratch using powershell.  Since the default groups are missing, I am trying to add them but I am getting an error.

PS C:\> $sp_web.CreateDefaultAssociatedGroups("A1\admin_ck","","")
Exception calling "CreateDefaultAssociatedGroups" with "3" argument(s): "User cannot be found."
At line:1 char:38
+ $sp_web.CreateDefaultAssociatedGroups <<<< ("A1\admin_ck","","")
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

Didn't find too much info on google, so any help is welcome