paramiko combine stdout and stderr

I am trying to combine the output of stdout and stderr. My belief is that this can be done with the set_combine_stderr() of a Channel object.

This is what I am doing:

SSH = paramiko.SSHClient()
#I connect and everything OK, then:
chan = ssh.invoke_shell()
chan.set_combine_stderr(True)
chan.exec_command('python2.6 subir.py')
resultado = chan.makefile('rb', -1.)

However, I get the following error when I try to store the result (last line above, chan.makefile() ):

Error: Channel closed.

Any help would be greatly appreciated