I've noticed the following behaviour on some of our curve sketching problems:
We'll ask for intervals of increase/decrease (or concave up/down), and of course, there are cases where a function is never decreasing (for example), so the correct answer for that interval is "DNE".
The trouble is that DNE is understood as a numerical value, even when the context is set to interval. So if a student does not enter DNE for their answer, the feedback message they get warns them that their answer is not a number (and looks like an interval).
This mostly confuses students, except for a small fraction who have clued in that this is a sign that WeBWorK is looking for DNE as the answer, who then get more of a hint than we might have intended.
Short of writing custom feedback for all these problems, is there a way to produce a more useful feedback message?