In Burch v. Children's Hospital of Orange County Thrift Stores, Inc. (2003) 109 Cal.App.4th 537, the plaintiff made an unapportioned $ 50 million section 998 offer to the four defendants. An unapportioned offer to multiple defendants has been held invalid unless the defendants were vicariously liable or jointly and severally liable. (Burch at pp. 545-547.) The complaint did not allege that the defendants were vicariously liable for each other's acts or jointly and severally liable. The defendants did not accept the offer. In advance of trial, the defendant stipulated to liability and to joint and several liability. The action was tried solely on damages, and the plaintiff recovered more than $ 50 million. She sought to recover her expert witness fees and prejudgment interest. Like defense costs under section 998, prejudgment interest under section 3291 is mandatory rather than discretionary. The trial court rejected her request for fees and interest on the ground that the unapportioned offer was invalid and did not fall within the exception for vicarious or joint and several liability because, at the time of the offer, liability was not alleged to be vicarious and joint and several. (Burch at pp. 540-542.) On appeal, the plaintiff asserted that the stipulation to joint and several liability brought the offer within the exception to the invalidity of unapportioned offers. The Fourth District rejected this contention. The Court found that the stipulation was irrelevant and the fact that the complaint did not allege vicarious or joint and several liability made the unapportioned offer invalid. (Burch at p. 547-551.) The appellate court examined three cases involving nonapportioned offers to multiple defendants, and provided the following summary of their holdings: "'A plaintiff who makes a 998 offer to joint defendants having potentially varying liability must specify the amount plaintiff seeks from each defendant. Otherwise, there is no way to determine whether a subsequent judgment against a particular nonsettling defendant is "more favorable" than the offer.' 'Thus, a lump-sum settlement offer made to several defendants whose liability may be apportioned (i.e., not jointly liable) must state plaintiff's position as to each defendant's share or percentage of the settlement demand.' " (Burch, supra, 109 Cal.App.4th at p. 547.) In Burch, the plaintiff was a victim of personal injuries resulting from the collision of her car with a truck driven by an employee of one defendant. The employee was acting in the course and scope of his employment at the time of the accident. The employer leased the truck from a second defendant. The plaintiff served an unapportioned section 998 offer to all defendants, who did not accept the offer, and the jury awarded damages in excess of the amount of the offer. (Burch, supra, 109 Cal.App.4th at p. 540.) The appellate court concluded that the unapportioned offer was not valid under section 998 because at the time the offer was made (which was before the parties stipulated to joint and several liability), the complaint alleged several theories of negligence against each defendant, and under some of those theories all of the defendants would not be jointly and severally liable for the full amount of the judgment. (Id. at pp. 549-551.) As an example, the court noted that under the allegations of the complaint, the owner of the truck that collided with plaintiff's car could be found liable based only on its role as owner, in which case that defendant's liability would have been limited under Vehicle Code section 17151 to $15,000 per injury, $30,000 per occurrence, and $5,000 for property damages. (Id. at p. 550, citing Veh. Code, 17151, subd. (a).)