Could not retrieve sendmail macro "i"!. Please add it to confMILTER_MACROS_ENVFROM for better spamassassin results

I have followed the build your own email server from scratch guide, and am getting the following errors in my mail.err log.

Could not retrieve sendmail macro "i"!. Please add it to confMILTER_MACROS_ENVFROM for better spamassassin results

Anyone have any idea what this is about and how to fix it?