Why I can't install a new domain in a new forest?

I try to install Active Directory & DNS on Windows 2003 in silent mode.
I want a new domain in a new forest. the win2k3 will be the DC, it is the root node.
My answer file is:

-----------------------------------------------------------------------------------------
[DCInstall]
NewDomain = Forest
AutoConfigDNS = Yes
SiteName = mydomain
ReplicaOrNewDomain = NewDomain
InstallDNS = Yes
SetForestVersion = Yes
SafeModeAdminPassword = xxxxxx
ReplicaDomainDNSName = mydomain.com
ReplicateFromMedia = Yes
NewDomainDNSName = mydomain.com
DomainNetBiosName = mydomain
AllowAnonymousAccess = No
CriticalReplicationOnly = No
DisableCancelForDnsInstall = No
DNSOnNetwork = No
IsLastDCInDomain = Yes
RebootOnSuccess = Yes
RemoveApplicationPartitions = Yes
ReplicaOrMember = Replica
-----------------------------------------------------------------------------------------

But when I execute dcpromo /answer:"c:\myfile.txt", it always popup a dialog and ask me to input User/Passwd/Domain. Why? This would happen only if NewDomain=Tree or Child.
Anybody can help me out?
Thank you.