Cal. Unemp. Ins. Code § 987

Each employer shall be liable for any and all contributions required to be made by his workers on account of wages which he has paid to them regardless of whether or not he has deducted the contributions from the workers’ wages at the time they were paid, but no employer shall be liable for worker contributions required on behalf of himself or of any of his employees with respect to wages paid while there is in effect at the time the wages were paid a rule or regulation or interpretation of the director or of the department that such wages were not subject to such contributions.