I'm having a hard time seeing any reason why setting 'score_sets' to 'ta' should cause this error.
However, I can reproduce the error by doing just what you say (see below)...
I'll keep looking,
Jason
Warning messages
Use of uninitialized value $primarySortField in hash element at /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/ProblemSetList.pm line 479.<br />Use of uninitialized value $secondarySortField in hash element at /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/ProblemSetList.pm line 480.<br />Use of uninitialized value $secondarySortField in string eq at /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/ProblemSetList.pm line 483.<br />Use of uninitialized value $secondarySortSub in sort at /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/ProblemSetList.pm line 486.<br />
Error messages
Undefined subroutine in sort at /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/ProblemSetList.pm line 486.
Call stack
The information below can help locate the source of the problem.
in WeBWorK::ContentGenerator::Instructor::ProblemSetList::body called at line 152 of /usr/local/webwork/webwork2/lib/WeBWorK/Template.pmin WeBWorK::Template::template called at line 494 of /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator.pmin WeBWorK::ContentGenerator::content called at line 198 of /usr/local/webwork/webwork2/lib/WeBWorK/ContentGenerator.pmin WeBWorK::ContentGenerator::go called at line 371 of /usr/local/webwork/webwork2/lib/WeBWorK.pm