Life insurance back charge for premiums after 14 months

I signed up for some additional life insurance through my work about 14 months ago. They said for $500k additional life insurance it is $9 a month. So I agreed to sign up. Now my work is saying the insurance company made a mistake and it should have been $45 a month and i owe the difference $504 in back charges. My company is saying insurance will not wave this charge so i have to pay. Is this normal?