(a) If at any time the assets of a domestic mutual insurer are less than its liabilities and the minimum amount of surplus required of it by this title for authority to transact the kinds of insurance being transacted and the deficiency is not cured from other sources, its directors shall levy an assessment only upon its members who at any time within the 12 months immediately preceding the date notice of the assessment was mailed to them held policies providing for contingent liability; and the members shall be liable to the insurer for the amount so assessed.

(b) The assessment shall be for the amount necessary to cure the deficiency and to provide a reasonable amount of working funds above the minimum amount of surplus; but the working funds so provided shall not exceed 5 percent of the insurer's liabilities as of the date as of which the amount of the deficiency was determined.

(c) In levying an assessment on policies providing for contingent liability, the assessment shall be computed on a basis of premium earned on the policy.

(d) No member shall have an offset against any assessment for which he is liable on account of any claim for unearned premium or loss payable.