This means that if you want to add to numbers, and then send the result to Add Motion trick, it wont work.

It literrally says it needs a number constant instead of number so it is not a bug. This could be nice ferature but I really myself wouldn't find any sense for it being a ferature as you would literrally die with high enough numbers. http://puu.sh/n5373/cc0e5a656a.jpg

It's intentional because that constant determines cost.