samba - Apr 2004 - smbclient not working with "-N" option, but OK with "-U" option

Why does this fail with "NT_STATUS_LOGON_FAILURE"
   smbclient -L //zero.private.local -N

and yet this works fine:
   smbclient -L //zero.private.local -U steev%paswd611

FYI also I have no trouble at all mapping and using shares in winXP,
but am unable to browse the workgroup in network neighbourhood.

Thanks

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