If a document's value for field "foo" is null, and "foo" is in the field list for mongoexport --csv, mongoexport refuses to export that document. The error message is, "Invalid BSON object type for CSV output: 10." Better if mongoexport wrote an empty string or the string "null" instead.

Steps to reproduce:

In mongo shell:

> db.nullcsv.insert({"a":null})

Running mongoexport:

$ ./mongoexport --csv -c nullcsv -d test --fields "a"
connected to: 127.0.0.1
a
Invalid BSON object type for CSV output: 10

exported 1 records

The same thing happens with deprecated BSON types "DBPointer" and "undefined".

It's unclear what the appropriate action here should be, but a small improvement may just be to improve the error messages to explain better why the failure is happening.