Setup.bash and Arch Linux

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Hello, I'm a new Ros-user. I installed it with success, but I have a problem. When I give in input in my shell source ~/ros/setup.bash I obtain as answer this:

Traceback (most recent call last): File "<stdin>", line 17, in <module> File "<stdin>", line 17, in <listcomp> TypeError: 'dict_keys' object does not support indexing rospack could not be found, you cannot have rosbash features until you bootstrap ros

And nothing works. If I type the command again all works and I don't receive any error messages.