Failed: error marshalling metadata json for collection

We get the following when running a regular "mongodump" command against our 3 node replica set (v3.0.3 on WiredTiger). The full command we run is as follows:

"mongodump --host xxx.xxx.xxx.xxx --username adminUserHere --password aSecretPassHere --ssl --authenticationDatabase admin --sslPEMKeyFile /path/to/pem/db.pem --sslAllowInvalidCertificates --out /path/to/dump/mongoBackup"

The majority of the database gets backed up as expected, however on the same collection on each run we get the following error...

"Failed: error marshalling metadata json for collection `ourDb.ourCollection`: json: error calling MarshalJSON for type bsonutil.MarshalD: cannot marshal [

]: json: error calling MarshalJSON for type bsonutil.MarshalD: invalid character 'u' after object key"

Any ideas what would cause this error, the data looks ok when viewing it.

Thanks.