In any action brought to enforce an insurance policy subject to the provisions of this Article, any party claiming benefit under the policy may reply to the pleading of any other party against whom liability is sought which asserts as a defense, the failure to render timely proof of loss as required by the terms of the policy that such failure was for good cause and that the failure to render timely proof of loss has not substantially harmed the party against whom liability is sought in his ability to defend.  The issues raised by such reply shall be determined by the jury if jury trial has been demanded. (1973, c. 1391.)