In what year are deductible self-employment health care insurance premiums reported (if different: date of payment, or year of coverage)?

In Schedule C, Other Business Expenses, in what year are deductible self-employment health care insurance premiums reported?  I am in a situation where I had to buy my own health insurance for 2019 health coverage.  One of those premiums had to be paid in December 2018.  I was self-employed in the later part of 2018.  Can I take the December 2018 premium deduction in my 2018 taxes, or do I have to wait till 2019?