That was a typo in the MathObject Matrix package, and it was fixed nine months ago, and the fix was included in pg version 2.8. If you are running an older version, you might want to update the pg/lib/Value/Matrix.pm file to the current version. You should be able to do that without updating everything else, but I would recommend updating to the latest release, as there are a number of important changes and bug fixes.
Remember that you need to restart Apache after updating this file.
WeBWorK Problems
having trouble with $matrix->solve_LR
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