Can a general contractor with a B-license do a sheet rocking project with out holding a C-9 drywall license?

I am a licensed general contractor in San Francisco. I provided a customer a quote on drywall repairs. The customer signed a contract and we are in the middle of completing the project. I received a letter from the customer stating that the California state board of contractors said that we were not able to perform this duty with a B-license, we needed a C-9 drywall license. Is this correct that we can not do drywall installation with a B-license.