The initial import on existing index

[tuser4198]

my es version is 1.4.2,mongo_river is 2.0.9,mongo is 3.0.when I setup river on existing index,es just report error: Cannot import collection collection1 into existing index
setRiverStatus called with river_1 - INITIAL_IMPORT_FAILED
Started river_1
and the sync dosen't start at all.
I know that Initial import supports existing index from 1.7.2 version .how it works? dose river judge by itself? or should I use this option: skip_initial_import?
I just want to setup river on existing index.