'The Princess Bride' Star Cary Elwes Sued for Not Paying His Bill ... INCONCEIVABLE!

"The Princess Bride" star Cary Elwes has A LOT OF FANS ... but a debt collection company in CA is NOT one of them ... claiming the actor dropped the ball on paying his credit card bill, to the tune of $40k.

According to a lawsuit filed in LA County Superior Court last month ... Elwes is being sued by a company called Investment Retrievers Inc. for allegedly racking up $40,334.12 on a Wells Fargo credit card back in 2010.

In the docs, IRI says it tried to work with the actor to get it's cash back ... and at first Elwes was cooperative, handing over a $2,000 check ... but IRI claims that was the last it heard from him.

Fed up, IRI decided to sue Elwes for the missing dough -- a cool $38,334.12 -- plus interest and attorney's fees.