Is there a difference between Lookup based alerting and Limit based alerting?

Hello all, i have a funny effect on a "HTTP Push Data Advanced Sensor" the setup is realy easy TLS= no (HTTP) Port= 5050 Requestmethode = GET Token = xxxxxxxxxxxxxxx

The recieved values are 1 to 3 = Error 4 is OK and if the sensor send nothing PRTG set the value to 0

I build a Lookup with for the different values incl the value 0

****************************************************************
<?xml version="1.0" encoding="UTF-8"?>
  <ValueLookup id="xxx_App_Status" desiredValue="2" undefinedState="Error">
    <Lookups>
      <SingleInt state="Error" value="0">
        not reachable
      </SingleInt>
      <SingleInt state="Error" value="1">
        Stopped
      </SingleInt>
      <SingleInt state="Error" value="2">
        Errored
      </SingleInt>
      <SingleInt state="Error" value="3">
        Status unkown
      </SingleInt>
      <SingleInt state="Ok" value="4">
        Ok
      </SingleInt>
    </Lookups>
  </ValueLookup>
****************************************************************

This works fine
The sensor change to Error if the sensorchannel send the value 1,2,3
The sensor change to Ok if the sensorchannel send the value 4
But
if the sensorchannel revieve nothing and PRTG set the value to 0 the channel will set it correct based on the lookup to "red" but the sensor will not change to Error

If i setup the alterting based on limits
Upper Error Limit = 4
Lower Error Limit = 4
It works fine

For me it will be nice to use a Lookup to give more informations about the Errortyp can you give me a workaround for this?

alert alerting-based-on-limits lookup