[Dovecot] Setting "default_mail_env" Why is index->mailbox_path empty?

[Robert McLay]

I now have 

default_mail_env = mbox:~/mail:INBOX=:/var/spool/mail/%u

I now getting:

Apr 26 15:55:32 jedrik imap(mclay): open() failed with mbox file : No such file or directory

I tracked down where the error message is getting generated and added
some more printing:

In mbox-index.c I added:

int mbox_set_syscall_error(struct mail_index *index, const char *function)
{
	i_assert(function != NULL);

        i_error("index->mailbox_path: %%%s%%", index->mailbox_path);
        i_error("index->dir: %%%s%%", index->dir);
        i_error("index->filepath: %%%s%%", index->filepath);
        i_error("index->control_dir: %%%s%%", index->control_dir);
	index_set_error(index, "%s failed with mbox file %s: %m",
			function, index->mailbox_path);
	return FALSE;
}

This generates the following output.  It seems clear that the initial reading
of mbox is read correctly because index->dir and others have values derived from
it but for some reason mailbox_path is empty.  This is version 0.99.10.4 built with 
gcc 3.3.2.  Here is the bottom of running ./configure:

---------------------------------------------------------------------------
Install prefix ...................... : /vol/pkg/dovecot/0.99.10.4
File offsets ........................ : 64bit
Building with SSL support ........... : yes (OpenSSL)
Building with IPv6 support .......... : yes
Building with pop3 server ........... : yes
Building with user database modules . : static passwd passwd-file (modules)
Building with password lookup modules : passwd passwd-file shadow pam (modules)
---------------------------------------------------------------------------

Apr 26 17:06:51 jedrik imap(mclay): index->mailbox_path: %%
Apr 26 17:06:51 jedrik imap(mclay): index->dir: %/home/mclay/mail/.imap/INBOX%
Apr 26 17:06:51 jedrik imap(mclay): index->filepath: %/home/mclay/mail/.imap/INBOX/.imap.index%
Apr 26 17:06:51 jedrik imap(mclay): index->control_dir: %/home/mclay/mail/.imap/INBOX%
Apr 26 17:06:51 jedrik imap(mclay): open() failed with mbox file : No such file or directory
Apr 26 17:06:51 jedrik imap(mclay): index->mailbox_path: %%
Apr 26 17:06:51 jedrik imap(mclay): index->dir: %/home/mclay/mail/.imap/INBOX%
Apr 26 17:06:51 jedrik imap(mclay): index->filepath: %(in-memory index for )%
Apr 26 17:06:51 jedrik imap(mclay): index->control_dir: %/home/mclay/mail/.imap/INBOX%
Apr 26 17:06:51 jedrik imap(mclay): stat() failed with mbox file : No such file or directory

Any ideas?

Thanks