Wilfred/ag.el — pull request #35 by ShingoFukuyama (2014-02-08 18:23:59+00:00)
Title: Use -I instead of -d '\\n'

Thanks grep information, but Mac OSX's xargs doesn't have -d too...
However, I found the equivalent of -d '\n', -I exists ether GNU and UNIX.
It works fine without abnormally exits (in case, caused by blank space file name).