Can't set field names with Given users behat call
I didn't think this was the issue but I recently upgraded to PHP 7.  I downgraded to PHP 5.6 and the issue went away.  I created an issue on Drupal.org.

Turns out that I was using an older version of drupal-extension.  Upgraded to and the issue went away:

"drupal/drupal-extension": "^3.2",