EIN assigned by IRS to sole proprietorship.

Hi, 

I just got an letter from IRS, in which it informs it assigns to my sole proprietorship an EIN because it is required by Law.  As far as I know EIN is required only if I have employees which I do not, it is only me working in the business. Does anyone know why I was assigned an EIN, and if that is a common practice. I filed Schedule C via turbotax. On it there is only my ssn and the ein is missing. 

Thank you.