Cal. CCP § 431.50

In an action to recover upon a contract of insurance wherein the defendant claims exemption from liability upon the ground that, although the proximate cause of the loss was a peril insured against, the loss was remotely caused by or would not have occurred but for a peril excepted in the contract of insurance, the defendant shall in his answer set forth and specify the peril which was the proximate cause of the loss, in what manner the peril excepted contributed to the loss or itself caused the peril insured against, and if he claims that the peril excepted caused the peril insured against, he shall in his answer set forth and specify upon what premises or at what place the peril excepted caused the peril insured against.