git - Assumed unchanged file changed on pull -

i have file config.properties contains information needed comunication webservice (ie api token etc.) , don't want information become public.

i made properties empty , file looked this:

accesstoken= domain= 

then issued git update-index --assume-unchanged config.properties , committed file.

after filled properties information needed file looked bit this:

accesstoken=xxxxxxxxxxxxxxx domain=http"//xxx.xxx.xx/xx 

when ran git status after file didn't list changed, far good.

did same other users contributing project , worked fine.

but here's problem: when pull file gets 'reset' it's 'original' state (ie 'empty' file) , end again:

accesstoken= domain= 

what have done wrong? or idea of git update-index --assume-unchanged wrong?