Attribute mapping error prevents successful database connection

When mapping Drupal profile fields to phpList attributes, leaving a data element blank results in a database connection error. The error page includes a blank error message, suggests that the database is not set up, and directs implementer to documentation for initial implementation of Drupal. Given the lack of editing and clues as to source of the error, I chased my tail for hours, thinking it was a password or security issue.

Quick implementer fix is to edit the "variable" table entries for the source and target attributes.