Calculation Engine fails with ANE

[Doc-Saintly]

I'm getting a System.ArgumentNullException when trying to use the calculation engine. I've put the stack trace below. According to this page it should be as simple as:

// Add an expression to the calculation engine as "a"
engine.Add("a", "x * 2", context);

// Add an expression to the engine as "b"
engine.Add("b", "y + 100", context);

// Add an expression at "c" that uses the results of "a" and "b"
engine.Add("c", "a + b", context);`

I am adding many variables to the engine and then trying to use them in dynamically compiled expressions. Is this supported? I have also tried using the Engine to calculate values using my custom function, but it throws an exception. Are these things not allowed to mix?

stack trace.txt