It works in some levelsets. For example, Blox 2, A JOB 2, Under the water.
When I choose any level in Level Number menu, it switches to level 000 and you can play empty (or secret) level when first level in levelset is 1 (not 0).
Examples:
Under the water (levels 1-6)
A JOB 2 (1-51)
LEVELNR Bug
Moderators: Flumminator, Zomis