We are trying to track down a particularly difficult bug that , intermittently, prevents users from completing an assignment.
In short, your code might be correct , but the assignment does not trigger as correct.
We are working on tracking this down, but until we patch this bug, there is one way to fix the issue. Just reload the page and try the assignment again with the exact same code! (So make sure you save your code before reloading)
We are publishing this post because we have been getting emails about it. Rest assured we will fix it, but until then please try a reload.