LAB §5705: The burden of proof rests upon the party or lien claimant holding the affirmative of the issue.

The burden of proof rests upon the party or lien claimant holding the affirmative of the issue. The following are affirmative defenses, and the burden of proof rests upon the employer to establish them: (a) That an injured person claiming to be an employee was an independent contractor or otherwise excluded from the protection of this division where there is proof that the injured person was at the time of his or her injury actually performing service for the alleged employer. (b) Intoxication of an employee causing his or her injury. (c) Willful misconduct of an employee causing his or her injury. (d) Aggravation of disability by unreasonable conduct of the employee. (e) Prejudice to the employer by failure of the employee to give notice, as required by Sections 5400 and 5401.