When using the segments functionality, The following error may be shown.

The application has experienced a system error. The reported error message was: Mailchimp returned an error: Each criteria row must contain the keys "field" (merge3), "op" (eq) and "value" ().

The happens when the segment criteria is similar to the following:

"ConsId is blank"

This is shown below:

The workaround is to do the following:

This is a problem as of v2.0.20 although a MailChimp fix may make this and earlier version work.