I think that the cause of this behaviour is the Mathematical definition of interval.

An interval [ a , b ] is defined as (free definition) "Every number greater than a and lesser than b" (definition). So, if a=2 and b=1, there's no number with such condition. That's a valid definition for every set with a ">" operator, so it could work with any set (class) with ">" defined and a valid "generator" (++) (This last statement is only theory, someone with time to test).