1099-MISC question and timing of receiving payment via US mail

I started a consulting position and set up a individual LLC.  I have a question about the 1099 MISC and how I should report the income.  My client sent me payment for Dec on Dec 31 (I have a postmark on the envelope) so I received the check on Jan 3, 2020.  Should I report that as income in 2019 or 2020?  I use the cash accounting method so I believe it should be reported as 2020 income however how do i reconcile the difference with the 1099 MISC form?