What is the standard of review for denying a motion to compel arbitration?

There is no uniform standard of review for evaluating an order denying a motion to compel arbitration. [Citation.] If the court's order is based on a decision of fact, then we adopt a substantial evidence standard. [Citations.] Alternatively, if the court's denial rests solely on a decision of law, then a de novo standard of review is employed. [Citations.] (Robertson v. Health Net of California, Inc . (2005) 132 Cal.App.4th 1419, 1425, 34 Cal.Rptr.3d 547.)