git subtree error "fatal: refusing to merge unrelated histories" -

i'm trying figure out how 'git subtree' works. i've followed directions on this page, error trying merge subtree project in own repo ('step 2'): fatal: refusing merge unrelated histories.

i've read this post, , when use --allow-unrelated-histories option, seems work fine. however, i'm not sure whether should use this...my impression whole point of subtrees have unrelated histories within 1 repository, feels strange have add option. should add nevertheless, or doing wrong?

i'm using git v2.9.3 on osx 10.11.6

this bug in git, fixed in https://github.com/git/git/commit/0f12c7d4d175bb0566208213b1b340b6794f305c

see also: https://www.spinics.net/lists/git/msg282340.html