A group health insurance policy shall contain a provision that the validity of the policy shall not be contested except for nonpayment of premiums, after it has been in force for 2 years from its date of issue; and that no statement made by any person covered under the policy relating to insurability shall be used in contesting the validity of the insurance with respect to which such statement was made after such insurance has been in force prior to the contest for a period of 2 years during such person's lifetime nor unless it is contained in a written instrument signed by the person making such statement; provided, however, that no such provision shall preclude the assertion at any time of defenses based upon the person's ineligibility for coverage under the policy or upon other provisions in the policy.