HP 30b: Bug in Student function?

[kakima]

OS 4 5 2010 1

I also get the wrong answer if I do 5 1.677 STO 1 MATH etc.
Also the wrong answer with 5 RCL 1 MATH etc.

Now try it with a negative value for t.
5 -1.677 MATH etc gives the right answer.
Doing anything after the 7 (two swaps, STO, etc) gives Invalid Input.

This also seems to happen with other values for df and t (obviously, I haven't tried all possible combinations) except when df == t.