C# compiler complains that abstract class does not implement interface?

I have a nice interface, and I want to implement one member of it in a base class so the clients can derive from the base class and have less boiler-plate to write. However, even though declared abstract, the compiler complains that the class does not implement the interface?!? How is this supposed to be done?

Code in question:

public interface ITaskDefinition
{
    ITask CreateTask(TaskId id);
    string Name { get; }
    bool HasName { get; }
}

public abstract class TaskDefinitionBase : ITaskDefinition
{
    private string name_ = null;

    public void SetName(string name)
    {
        name_ = name;
    }

    public string Name
    {
        get
        {
            return name_;
        }
    }

    public bool HasName
    {
        get
        {
            return name_ != null;
        }
    }
}

and the error I get is:

ITaskDefinition.cs(15,27): error CS0535: 'NetWork.Task.TaskDefinitionBase' does not implement interface member 'NetWork.Task.ITaskDefinition.CreateTask(NetWork.Task.TaskId)'