Each policy of life insurance shall contain a provision that the policy is incontestable, except for nonpayment of premiums, after the policy has been in force during the lifetime of the insured for 2 years after its date of issue. The incontestability provision described in subsection (a) of this section does not apply to policy provisions that relate to disability benefits or to additional benefits for death by accident or accidental means. A provision in a policy of life insurance that provides that the policy is incontestable after a specified period: precludes only a contest of the validity of the policy; and does not preclude the assertion of defenses based on provisions in the policy that exclude or restrict coverage, whether or not the restrictions or exclusions are excepted in the provision.