4/20 6:56 AM PT -- A rep for Calvin tells us, "The claims filed against Calvin Harris are absurd and without merit. We are confident that the Court will dismiss the frivolous lawsuit."

Calvin Harris is being sued by a blind man who claims the DJ screwed him over in a super-expensive home deal.

Harry Moscatel claims he leased Harris' home above the Sunset Strip in West Hollywood last year for $35,000 a month. He put down a $70,000 security deposit and moved in ... according to the docs.

Moscatel's attorney, Scott Doonan, tells us ... the house had problems from the beginning, including hot water and heating issues, faulty doors and a sound system that never worked right.

We're told Moscatel and his family bailed on the home, and he now claims the lease agreement was invalid ... in part because he's a 79-year-old blind man and protected by the Americans with Disabilities Act. The lawsuit does not explain why his disability would allow him out of the lease.

The house was once the love nest of Calvin and Taylor Swift when the 2 were an item.

Moscatel makes other technical arguments about the lease and is asking a judge to declare it null and void. He's also preparing to sue Harris to get a portion of his rent and his security deposit back.

BTW ... Moscatel made news last year after a hoverboard caught fire and burned his multi-million-dollar mansion down. Presumably, that's why he rented Calvin's pad.

We reached out to Calvin's people, so far no word back.