"x509: certificate signed by unknown authority" when the origin uses a internal cert

My origin server runs with a certificate signed by the root CA of my organization. I don’t know how to make cloud flared trust our organization root CA. The error I have is:

ERRO[0000] unable to connect to the origin error="Get https://***: x509: certificate signed by unknown authority"

I tried --origin-ca-pool but it did not prevent the error.