Error running MapReduce, which writes result to another db with authorization.

Have problem when try run MapReduce, which writes result to another db.

Both db have authorization.

MR out section:
'out' :

{ 'merge' : $_outputCollection, 'db' : $_anotherDB, 'sharded' : true }

Fails with error: MR parallel processing failed:
{ result: "tmp.mrs.views_raw_1348067821_5", errmsg: "exception: splitVector failed:

{ errmsg: "need to login", ok: 0.0 }

", code: 15921, ok: 0.0 }

How I must login with MapReduce task?
May be need change 'db' to array

{'name':'$_anotherDB', 'user':'login', 'password':'pass'}

?