Ask the previous tenant for his copy of a signed current lease - that should have been something you got during due diligence of taking over the space at that time. Did you not confirm what the actual lease was and just took an unsigned copy as fact? If so, it is very possible that they were under a MTM least from the expired one in 200 since you don't have a signed copy.

I work for a property management company and will tell you the lease generally isn't valid if both parties have not signed it - so you need to be looking for a signed copy. If it doesn't exist, then the landlord isn't bluffing, rather your previous tenant was.