Difference in behavior with ninja when not specifying a target

[GrigorenkoPV]

rule touch
    command = touch ${out}

build a: touch
build b: touch

$ ninja -v
[1/2] touch a
[2/2] touch b
$ n2
n2: error: no path specified and no default
$ readlink /usr/lib/n2/bin/ninja 
/usr/bin/n2
$ /usr/lib/n2/bin/ninja 
n2: error: no path specified and no default