This issue is directly related to issue #6697. The first problem is that the builtin input() function doesn't check that _PyUnicode_AsString() result is not NULL.
The second problem is that io.StringIO().encoding is None. I don't understand why it is None whereas it uses utf8 (it calls TextIOWrapper constructor with encodings="utf8" and errors="strict").
I will be difficult to write an unit test because the issue only occurs if stdin and stdout are TTY: input() calls PyOS_Readline(stdin, stdout, prompt).
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@gruszczy: You're patch is just a workaround, not the right fix. The problem should be fixed in input(), not in PyUnicode methods. _PyUnicode_AsString() expects an unicode argument, it should raise an error if the argument is None (and not return a magical value).