EDITED TO ADD:
In fixing the bug listed below we realized that we no longer need the fallback mechanism causing the problem at all since we don't support 2.6/3.4 mixed mode replica sets.

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If you attempt to authenticate with SASL mechanism PLAIN to a server that does not have PLAIN enabled using authenticationMechanisms you receive the incomprehensible error:

Error: Missing expected field "mechanism"

MongoDB Enterprise > db.auth(

{user:"u1",pwd:"123qw",mechanism:"PLAIN",digestPassword:false}

)
Error: Missing expected field "mechanism"

Improve this error message to indicate what it going wrong.