JSHint error on Ember build

I am in the process of upgrading an Ember application from version 2.4.3 to 2.9.1. The application builds successfully, but I get a list of 42 versions of the following JSHint error; each starting with a different file name

unit/services/tracker-test.js: line 0, col 0, Incompatible values for the 'esversion' and 'esnext' linting options. (0% scanned).

My .jshintrc file contains

...
"esnext": true,
"esversion": 6,
...

What is coursing the errors?