Do we have to defend a third party whose negligence caused our worker’s injury?

Q. One of our employees was badly injured when he tripped over equipment that had been left by our office building’s maintenance workers. Workers’ comp covered his medical bills and lost wages, but he also sued the building’s owners for negligence. Now we have a letter from the owners demanding that we defend them in the lawsuit and telling us we are responsible. How can that be? I thought we were protected by workers’ comp.