get_partial_grade returns full marks for a cell which should receive 0

[alexgbrandt]

During autograding I noticed a few submissions brought up a warning including "expecting output to indicate partial credit and be single value between 0.0 and max_points." (Side note, in the warning message should "and be" -> "to be a" ?)

I thought this was weird, because I had written the test cases with the intention of using partial credit in this way, and that this warning only appeared when autograding some submissions.

After some digging I discovered those few submissions actually should be getting 0, but instead received full marks. I believe the issue is just in this following line. Should it be >= 0.0?

nbgrader/nbgrader/utils.py

Line 87 in 5a81fd5

if (grade > 0.0):