I got a mail from IRS saying that I may be liable for self-employment tax. This is from 1099-MISC which I got from commission refund of buying house.

I got a mail from IRS saying that I may be liable for self-employment tax. This is from 1099-MISC income $40,200 which I got from commission refund of buying house which is a one time thing. How do I explain that I am not liable for self-employment tax.