share permissions - an object named Administrators cannot be found

Using 2008 R2 on Hyper-V:  I've created a share on the file server, which is not the domain controller.  When I try to add the domain\Administrators group to 'Share Permissions', it says "an object named Administrators cannot be found".  If I [Check Names] with 'A', it finds Anonymous and Authenticated Users, but it doesn't find Administrators.  It indicates the correct domain.  What could be wrong?

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