If you pass the interconnect function a string instead of a list, it treats the string as a list (of characters). For example:

P1 = ct.rss(2, 1, 1)
P1_iosys = ct.LinearIOSystem(P1, inputs='u1', outputs='y1')
P2 = ct.rss(2, 1, 1)
P2_iosys = ct.LinearIOSystem(P2, inputs='y1', outputs='y2')
P_series = ct.interconnect([P1_iosys, P2_iosys], inputs=['u1'], outputs=('y2'))

generates the following error:

ValueError: could not find signal y

The problem appears to be that ('y2') is the same as 'y2' in Python and is being interpreted as ['y', '2']. Changing this to outputs=('y2',) or outputs=['y2'] fixes the problem, but it is still a bug.