To reproduce:

• Bring up a 2.6 node
• Insert a user
• Manually set the admin.system.version doc to have

  { currentVersion: '3' }

• Run an authentication command, i.e usersInfo

The error message is a little misleading - it reports that the current schema version is 0, instead of the wrong type. It is conceivable that while running a downgrade someone could mistakenly set the value to a string type and get this error.

A js script to reproduce the error is attached.