Coverdell ESA Basis

I had a distribution from my Coverdell ESA in August 2019. TurboTax asks three questions under the "This Coverdell ESA" section. I'm unable to answer the 2nd question:

 

"Basis in this Coverdell ESA Account as of December 31, 2018"

 

I'm unsure what this means. Thoughts/guidance appreciated.