Problem using discretize function

I have a column "Net Pickups" in a table which contains integers from about -100 to 100. I want to discretize it to create a categorical variable "Demand" such that:

· Net Pickups < 0: ‘Low’

· 0 <= Net Pickups < 15: ‘Medium’

· Net Pickups >= 15: ‘High’

However, I'm getting the error that my 'values' vector should have the same length as my number of bins i.e. 3. What am I doing wrong? Here's a ballpark of the code I think should be used

values = [min(trainData.NetPickups) 0 15 max(trainData.NetPickups)]

discretize(trainData.NetPickups,3,values,"categorical",{'Low','Medium','High'})