How can I claim back tax paid on imputed income when my spouse was wrongly registered by my employer as domestic partner?

I noticed that my employer wrongly registered my spouse as domestic partner (DP) for the whole of last year. As a result the insurance premium for my spouse was considered as extra taxable income (imputed income). The W2 has already been issued and my employer says there's nothing he can do on his side. How can I claim the extra tax I paid?