Procmail/Smartlist save to directory problem

I have a recipe in Smartlist to save certain mails to a directory in MH 
format (yyyy/.).  I had the Smartlist to create this directory upon this 
recipe.  Everything went fine.

In addition, I need to also save certain mails into this same directory in 
the MH format from recipes in my personal .procmailrc (/path/to/yyyy/.).
I made sure my account has proper group permissions (770; my account is 
on its group; I manually tested) to write into this directory. However, 
the process of my account .procmailrc generates following errors:

  procmail: Unable to treat as directory "/var/slist/xxx/yyyy"
  procmail: Assigning "LASTFOLDER=/var/slist/xxx/yyyy"
  procmail: Opening "/var/slist/xxx/yyyy"
  procmail: Error while writing to "/var/slist/xxx/yyyy"

In the meanwhile the Smartlist saves into the MH directory still fine.  I 
wonder what could be the problem?

Zhiliang