Get generic Type out of ICollection

I'm using the following method but on a Dictionary it returns the TKey. Dictionary implements ICollection<KeyValuePair<TKey,TValue>> so how can i get KeyValuePair<TKey,TValue>?

public static Type GetCollectionGenericType( this Type type )
{
    foreach( Type interfaceType in type.GetInterfaces() )
    {
        if( interfaceType.IsGenericType &&
            interfaceType.GetGenericTypeDefinition() == typeof( ICollection<> ) )
        {
            return type.GetGenericArguments()[ 0 ];
        }
    }
    return null;
}