I am trying to insert a line into a coffee script using sed and it looks as if it is ignoring my first tab character.

The file I want to insert into has a line like this:

    karmaServer.start

and I want to insert a line after it like this:

    karmaServer.start
        flags: ['--no-sandbox']

So I wrote a sed script that I think should work

sed "/^\t\tkarmaServer.start/a\t\t\t\tflags: ['--no-sandbox']" build/unit-test.coffee 

although the result is quite odd. I get the following

    karmaServer.start
t       flags: ['--no-sandbox']

And I don't understand why it starts the line with a t character or how to get rid of it.

any ideas?