This is an issue in 1.9.13 but the report has been replaced in Moodle 2+
I imagine it won't be patched for 1.9.13+ but can provide a github patch if devs want it.
To fix, open up /mod/quiz/report/analysis/report.php and modify the following on line 157 and replace
if ($resp) {
with
if ('' !== $resp) {
Diff is attached.