The attached error message is not intuitive. The error occurs because the server is version 3.6 and not 4.0 and the agg operator was introduced in 4.0. However, the error message seems to suggest that a parsing error occurred in Compass.

Unrecognized expression `$toInt`

One way to improve these error messages would be to prepend a reference to the database server for those errors originating from the server.

Database server reported the error: Unrecognized expression `$toInt`