Currently, when the validator expression contains {$type: 'number'} (or, equivalently, the $jsonSchema keywords type or bsonType), the 'expectedTypes' field will report the types that are being aliased (i.e. double, long, int, decimal) and not the 'number' alias specified by the user. The error message might be clearer for $type if the 'number' alias is reported under 'expectedTypes' instead.