Does a residual infection below molar need tooth to be extracted before Invisalign?

I have a historic root canal in my first lower molar on one side (25 years+). It has caused me no problem at all, and looks totally fine. My dentist says I need the tooth extracted before they will do invisalign, as on xray there is some residual infection below the tooth. Is this is a problem, or specific to this dentist. Thanks,