bsondump -vvvvv report.bson
file size: 32770
INVALID OBJECT - going to try and print out
size: 32770
error: bson length doesn't match what we found in object with unknown _id
...
object up to error: { iwrite: 1, database: [ 0 ], sections: { groups:
assertion: 10334 BSONObj size: 0 (0x0) is invalid. Size must be between 0 and 16793600(16MB) First element: EOO

Assertion about says size must be between 0 and 16MB. The size of the
document is 0. So, if 0 is not valid, then the message is invalid. Otherwise, there is a bug in parsing the BSON.

Workaround was not to dump object to file if it is 0.