Question about spouse on my healthcare policy through work.

I feel like an idiot posting this, but:

My husband is on my employer’s insurance policy. Every fall, my employer runs an eligibility verification check on dependents on the plan. While reviewing the criteria, I realized that I should have been paying a spousal surcharge (husband became eligible for healthcare through his employer between 6-9 months ago – prior to that his job didn’t offer insurance).

I was not aware of the surcharge until I reviewed the eligibility criteria for working spouses last week (but I’m sure I electronically ticked a box or signed something along the way that said I was). I assumed that because I pay the premium for myself and one other person, we were fine. My job also stipulates that we need to inform them of any changes to a dependent’s health insurance/eligibility status (qualifying life events) within 30 days of the change. I was also not aware of this – I usually just skim over the fine print and sign the document.

I plan to contact HR and plead ignorance/offer to pay the surcharge I should have had deducted from my checks since my husband became eligible. I honestly didn’t realize I was supposed to be paying an extra $100 a month and wasn’t trying to screw my employer out of money. Am I at risk of being terminated? Is there anything I can do or say to make the situation better on my part?