Should be fixed in 0.2.2; one of the mods you are using defines an item where item.stack_size is not a number. I'm not entirely sure what I should do with that. Presumably it is perfectly valid, as the game would have objected to that item in that case. That said, I don't know if Factorio will treat that item as if it has a stack size of 1 (the default) or if it parses the value as if it was a number, and uses that for the stack size.
