getLastError response for failed index build contains extra copy of "err" field

db.foo.ensureIndex({a:"blah"});

printjson(db.getLastErrorObj());

{

	"err" : "Unknown index plugin 'blah' in index { a: \"blah\" }",

	"code" : 67,

	"n" : 0,

	"connectionId" : 2,

	"syncMillis" : 0,

	"writtenTo" : null,

	"err" : "Unknown index plugin 'blah' in index { a: \"blah\" }",

	"ok" : 1

}