Because the unit tests are not to "check types", they are to check that incorrect values (e.g. a string instead of a list of strings) do not occur. They are no

Because the unit tests are not to "check types", they are to check that incorrect values (e.g. a string instead of a list of strings) do not occur. They are no different from other kinds of incorrect values, like attempting to cluster an odd number of items into pairs.