Tricky NullPointerException

For my Computer Science course, we're making a program that simulates a vending machine business. Everything seems not to have any errors, but then this line in the driver runs,

VendingMachine pv0001 = new SnackMachine(19.00); 
pv0001.addItem(new Chips("Doritos"));

it returns this error

Exception in thread "main" java.lang.NullPointerException
    at SnackMachine.addItem(SnackMachine.java:35)
    at Driver.main(Driver.java:9)

Which is apparently stemming from the declaration of this if statement the SnackMachine code:

public boolean addItem(VendingMachineItem item)
    {
       if((item instanceof Snack) && (inventory.size() < maxCapacity))//error on this line
       {
           inventory.add(item);
           return true;
       }else{
           return false;
       }
    }

I can't seem to figure out what the issue is, can anybody help me?