"too many values to unpack" error when adding a regression test to GM

I am using matrix.py to add a new regression test to the Grammar Matrix:

python matrix.py regression-test-add choicesfilename testsuitename

and I am seeing this error:

Error adding regression test
Too many values to unpack

What is the reason for this and how do I fix it?

This will happen if there is a missing newline at the end of gmcs/regression_tests/regression-test-index file. Sometimes when you add a new test, if the newline character got deleted from regression-test-index, the new test name gets appended incorrectly. Check that the end of regression-test-index looks correct in that respect.