C# and AOP - AOPAlliance (Aspect-oriented programming) how does this work

I just had a very interesting experience with AOP in C#. I have a function with a return type List which is being intercepted and that's all well and good. However the interceptor function is a validator style function and can prevent the real function by being called and returning the boolean false.

So the code looks a little bit like this:

List<Update> updates = Manager.ValidateAndCreate();

// protected void Save(List<Update> updates) { ....
Save(updates);

The Method Interceptor looks like the following

public class ExceptionAdvice : AopAlliance.Intercept.IMethodInterceptor {

    public object Invoke(AopAlliance.Intercept.IMethodInvocation invocation) {

        if (isValid(invocation)) {
            return invocation.Proceed();
        } else {
            return false;
        }
    }

    private bool isValid( ...
 }

Now after validation fails the value of updates is actually a boolean not a List, I thought there would be some kind of runtime error here but there was not, so:

updates.GetType().Name == "Boolean"

But:

updates is bool == false

So save will still accept its mutated list of updates and will complain later on when you try to use it.

So how is this possible in a type safe language like C#? btw it's spring-aop.

Edit: Also this does compile and it does work i've stepped through it a few times now.