KindleGen gives Guide Title Empty warning

Environment: 
KindleGen via Kindle Previewer (probably cross-platform, but this was in Windows 7)
Problem: 

When opening an ePUB, the Kindle Previewer software will run KindleGen to compile the book. It may report that there are warnings. Looking at the details of the warnings, you may see "Guide title is empty. Item is ignored" in the list.

Status: 
Fixed
The Fix: 

This indicates that the title attribute is missing from an item in the <guide> section of the content.opf file in the ePUB file. To fix this, open the content.opf file, scroll down to the <guide> section, and examine each reference in that section. None of the title attributes should be blank, so put a title in each one that is blank. (If your content.opf file is generated automatically, you should insert the title content into the <title> in the <head> section of each document that has a blank <guide> title attribute.)