As the comment mentions, I just wanted a value that didn't change the appearance of the array contents too much. Using a zero meant that something like `233.78448486` would be shown, but using a fill value like 999999 would result in `2.3378448486e+2`. Using NaN as the fill value might well have the same property of preserving the "normal" 233.78448486 appearance. I don't really mind - the mask attribute has the mask information so this value doesn't need to be unique.