[Dovecot] Help with Virtual Plugin

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..