there is a check earlier in the code to confirm the array provided is not empty

Looking at your code, that check seems to happen too late. Why not take care of this case at the start of function?

It's most probably an exception thrown by at, when i is out of range of whatever the snail_map[leftRightCount] is. snail_map is not empty, but snail_map[leftRightCount] is smaller than i.