C# winform 西门子报错S7.net：The size of the class is less than 1 byte and therefore cannot be read

System.Exception
  HResult=0x80131500
  Message=The size of the class is less than 1 byte and therefore cannot be read
  Source=S7.Net
  StackTrace:
   at S7.Net.Plc.ReadClass(Object sourceClass, Int32 db, Int32 startByteAdr)
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()