I agree you shouldn't be seeing a rather inelegant traceback like that.
But the error you're getting is from the following three lines of code within CodeRunner:
$header = $resulttable[0]; $ishiddencolumn = -1; $n = count($header);
Since you're using a combinator template grader, the resulttable variable is taken directly from the output of your grader. It should be a list of lists, where the first list is the header row of the table. This should be a list of strings (the column headers), but the error message suggests it's actually just a single string.
However, if you're sure that your resulttable is indeed a list of lists of strings, please post back with an xml export of your question plus an export of its prototype.