Unable to prove Requires with IComparable<T>

Question

With the following code (from this SO question):

        public static bool Foo<T>(T value, T lowerBound, T upperBound)
        where T : IComparable<T>
        {
            Contract.Requires(upperBound.CompareTo(lowerBound) >= 0);

            return IsBetween(value, lowerBound, upperBound, 1);
        }

        public static bool IsBetween<T>(T value, T lowerBound, T upperBound, int test) where T : IComparable<T>
        {
            Contract.Requires(upperBound.CompareTo(lowerBound) >= 0);

            return false;
        }

The call to IsBetween gets flagged with the warning "Requires unproven: upperBound.CompareTo(lowerBound) >= 0", which should be proven by the Requires in the containing method.