这个错误意味着什么?考虑使用DataContractResolver或添加任何未知的静态类型

我正在使用EF, Web api 2，当我将我的网站部署到iis时，我得到这个错误

<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
Type 'System.Data.Entity.DynamicProxies.Category_31F98ED9DF233911D0B0BFA8D6969A7F549E4E0B5FAB4A4D16548D6E2A838BD0' with data contract name 'Category_31F98ED9DF233911D0B0BFA8D6969A7F549E4E0B5FAB4A4D16548D6E2A838BD0:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.
</ExceptionMessage>
<ExceptionType>
System.Runtime.Serialization.SerializationException
</ExceptionType>
<StackTrace>
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at WriteArrayOfCategoryToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , CollectionDataContract ) at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken) --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Web.Http.Owin.HttpMessageHandlerAdapter.<BufferResponseContentAsync>d__13.MoveNext()
</StackTrace>
</Error>

我的代码是这样的

public class DefaultController : ApiController
{
    public IHttpActionResult Get()
    {
        var dbContext = new MyDbContext("ConnectionString");
        return Ok(dbContext.Categories);
    }
}

编辑

   public class MyDbContext : DbContext
    {
        public MyDbContext(string nameOrConnectionString) : base(nameOrConnectionString)
        {
           // this.Configuration.ProxyCreationEnabled = false;
            Database.SetInitializer(new CreateDatabaseIfNotExists<MyDbContext>());
        }
        public DbSet<User> Users { get; set; }
        public DbSet<UnitType> UnitTypes { get; set; }
        public DbSet<UserStorage> UserStorages { get; set; }
        public DbSet<Category> Categories { get; set; }
        public DbSet<Role> Roles { get; set; }
        public DbSet<Storage> Storages { get; set; }
        public DbSet<StorageItem> StorageItems { get; set; }
    }

public class Category
{
    [Key]
    public Guid Id { get; set; } = GuidCombGenerator.GenerateComb();
    public string Name { get; set; }
    public virtual ICollection<StorageItem> StorageItems { get; set; } 
}