Procmail and mail rules

procmail is setup to looking in the users own .procmailrc file for their own server side filtering of emails.

I was playing with mine the other day and the file is becoming quite big and messy to read. I tried to set the INCLUDEC=$HOME/.mailrules to a directory and have all my seperate mails rules in seperate files like

.shops
.forums
.personal

etc, etc, but the mail log reports a failure like

procmail: Couldn't read "/home/<user>/.mailrules/"
and
procmail: Couldn't read "/home/<user>/.mailrules" (if slash not added)

Is there a way to split the rules files up or do they have to be in the one file?

Thanks.