Python 3.8.0a4+ (heads/master:870b035bc6, May 16 2019, 20:53:02) [GCC 9.0.1 20190402 (experimental) [trunk revision 270074]] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import sys >>> 'hi'[1::sys.maxsize] Objects/unicodeobject.c:14038:55: runtime error: signed integer overflow: 1 + 9223372036854775807 cannot be represented in type 'long int' 'i' >>> This is because unicode_subscript() performs an extra addition (cur += step) at the end of the for loop (which can overflow). The result of that final addition is not actually used. A patch to fix this issue was posted on #1621, but it seems that the patch has been abandoned. The bug is also described in detail in that issue. I have tweaked the patch and will open a PR.