WiX Votive Preprocessor Variable Not Finding Project Name

In WiX, the votive preprocessor variables allow the use of syntax such as $(var.ProjectName.TargetDir), where the ProjectName is the name of a project within your solution. However, you may get an error that the variable is undefined. If so, the most likely reason is that you need to add a reference to the project mentioned in ProjectName to the WiX project that is using the variable.

I got on a wrong track thinking that maybe WiX was confused by the fact that I had periods inside the project name. That, however, was a red herring. It would have complained about any project name.