Hi Alejandro,
The message itself is harmless -- to hide it, you can either configure PHP not to display notices or warnings to the browser, or you can apply the patch at
http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=2801 to correct the problem. Let me know if you need help applying the patch.
Regards,
Alec Smecher
Public Knowledge Project Team
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