The art market held its own last year, but the world’s chief auction houses are bracing for an unpredictable season ahead.

Christie’s International sold $7 billion of art last year, up 6% from 2017. The London-based company said on Wednesday it auctioned $6.3 billion in art, a 6% increase due in part to the $835 million estate sale of banking billionaire David Rockefeller and his wife, Peggy. The house sold an additional $653.3 million in art in privately brokered deals, up 7%. Its online-only art sales rose 20% to $86.6 million.