Empty <imprint> elements, which are valid in P4, show up sometimes because there's neither publisher nor date information for a publication (for instance, a Website for which no-one is explicitly taking responsibility). In P5, empty <imprint> elements are not allowed, so I had to add a template to the P4-to-P5 conversion code to trap for this. I'm now adding a date element that looks like this:

<date>n.d.</date>

with n.d. standing for "no date".