[Dovecot] Help with Virtual Plugin

[Joy]

Dear Experts,
                    I have configured virtual plugin for my setup
which is working as expected but after this dovecot lda is unable to
deliver message to any mailbox and showing following error:-


Error: user XXXXX: Initialization failed: Namespace 'virtual.':
Unknown mail storage driver virtual
Aug 28 12:46:27 lda(XXXXX): Fatal: Invalid user settings. Refer to
server log for more information.

Here is Namespace setting on my server:-


namespace {
        type = private
        separator = .
        prefix =
        inbox = yes
}

namespace {
  prefix = virtual.
  separator = .
  location = virtual:~/Maildir/virtual:LAYOUT=maildir++
}


Please let me know what could be the issue..