Mongoexport doesn't work with --fieldFile

$ mongoexport -c test -d test --fields x

connected to: 127.0.0.1

{ "_id" : { "$oid" : "5237218d6b8872341646d85f" }, "x" : 1 }

{ "_id" : { "$oid" : "5237218e6b8872341646d860" }, "x" : 1 }

{ "_id" : { "$oid" : "523721916b8872341646d861" }, "x" : 1 }

{ "_id" : { "$oid" : "523721966b8872341646d862" }, "x" : 2 }

{ "_id" : { "$oid" : "523721976b8872341646d863" }, "x" : 2 }

exported 5 records

$ cat fields.txt

x

$ mongoexport -c test -d test --fieldFile fields.txt

connected to: 127.0.0.1

{ "_id" : { "$oid" : "5237218d6b8872341646d85f" }, "x" : 1, "y" : 1 }

{ "_id" : { "$oid" : "5237218e6b8872341646d860" }, "x" : 1, "y" : 1 }

{ "_id" : { "$oid" : "523721916b8872341646d861" }, "x" : 1, "y" : 2 }

{ "_id" : { "$oid" : "523721966b8872341646d862" }, "x" : 2, "y" : 2 }

{ "_id" : { "$oid" : "523721976b8872341646d863" }, "x" : 2, "y" : 1 }

exported 5 records

$