Strange lock-up when reverse indexing iterables

[Knusper]

Reproducable the following leads to a rather long noticeable lock-up in my emacs:
I open a file ~/test.py in with anaconda mode. I enable company & company-anaconda (M-x company mode RET M-x company-anaconda RET). I start typing:

teststring = 'thisisiatest'
new_test = teststring[:-4]
2nd_test = teststring[:-1]

In the second line, after I type -1 emacs freezes for quite a long time.