What is the burden of proving impossibility or excuse in the context of a claim of force majeure?

The burden of proving impossibility or excuse is on the party asserting the defense. (See Butler v. Nepple (1960) 54 Cal.2d 589, 598-599, 6 Cal.Rptr. 767, 354 P.2d 239) Thus, even if we had concluded that the force majeure clause applied not only to the lease covenants but also to the conditions precedent in the habendum clause, we would nevertheless affirm the trial court's determination that the lease terminated on July 21, 1987, because appellants failed to prove that any of the circumstances set forth in paragraph 20 prevented their predecessors from operating a producing well as of the end of the primary term.