I'm trying to call an external web service from Apex.

The web service call returns an array of objects. So the generated Apex definition for the call is something like:

public class testWsdl {

    public class CalculationPort {

        public testWsdl.ComputedType[] calculate(DateTime start,DateTime end_x,DateTime now)
        ...

When I invoke it, I use the following code (declarations omitted for clarity):

  testWsdl.CalculationPort stub = new testWsdl.CalculationPort();
  testWsdl.ComputedType[] results = stub.calculate( start, end_x, now);

I get the following error on the second line:
  Non-void method might not return a value

I have no idea what this means. I've also tried the following code:
  testWsdl.ComputedType[] results = testWsdl.ComputedType[0];
  results = stub.calculate( start, end_x, now);

And I get exactly the same error.

I think I understand what the error means, that the variable results may be null after the call has completed.

My question is, how do I get rid of the error?