Using Contract.ForAll in Code Contracts

Okay, I have yet another Code Contracts question. I have a contract on an interface method that looks like this (other methods omitted for clarity):

[ContractClassFor(typeof(IUnboundTagGroup))]
public abstract class ContractForIUnboundTagGroup : IUnboundTagGroup
{
    public IUnboundTagGroup[] GetAllGroups()
    {
        Contract.Ensures(Contract.Result<IUnboundTagGroup[]>() != null);
        Contract.Ensures(Contract.ForAll(Contract.Result<IUnboundTagGroup[]>(), g => g != null));

        return null;
    }
}

I have code consuming the interface that looks like this:

    public void AddRequested(IUnboundTagGroup group)
    {
            foreach (IUnboundTagGroup subGroup in group.GetAllGroups())
            {
                AddRequested(subGroup);
            }
            //Other stuff omitted
    }

AddRequested requires a non-null input parameter (it implements an interface which has a Requires contract) and so I get a 'requires unproven: group != null' error on the subGroup being passed into AddRequested. Am I using the ForAll syntax correctly? If so and the solver simply isn't understanding, is there another way to help the solver recognize the contract or do I simply need to use an Assume whenever GetAllGroups() is called?