Can someone explain me what's the point of this kata?

what's up with the last test case? for some reason it doesn't return true although all other cases worked perfectly fine.

TypeError: Cannot read property 'length' of undefined
at tug_o_war
at testResult

I get this kind of error, although my code works, when I test it on http://eloquentjavascript.net/code/

Input: Quick brown fox jumps over the lazy dog ,
Expected Output: Rvjdl cspxo gpy kvnqt pwfs uif mbaz eph ,
Actual Output: Rvjdl cspxo gpy kvnqt pwfs uif mbаz eph

so expected output and actual output are identical, why it gives me an error?
all test cases work out fine, but this.
Is it some bug?