Fluent mapping of IDictionary

Given the following:

public class ClassA
{
    public IDictionary<int, ClassB> Details = new Dictionary<int, ClassB>();
    ...
}

public class ClassB
{
    public ClassA ClassA;
    public int MapKey;
    ...
}

I'm using fluent nhibernate automapping with the following overrides:

public class ClassAOverride : IAutoMappingOverride<ClassA>
{
    mapping.HasMany<ClassB>(x => x.Details).Inverse().AsMap(x => x.MapKey).Cascade.AllDeleteOrphan();
}

public class ClassBOverride : IAutoMappingOverride<ClassB>
{
    mapping.References<ClassA>(x => x.ClassA).Not.Nullable();
}

At runtime I get the following error from the "Configure" statement:

"The type or method has 2 generic parameter(s), but 1 generic argument(s) were provided. A generic argument must be provided for each generic parameter."

How can I make this work?