what it will return if key not found in lookup table

Hi Firneds

I have few odscodes in Lookup table named "AllowLT" .

Now I need to continue if I found value in lookup table or stop it. For that I have colleting the value form lookup table for input HL7 message OBR:21  value.

I know if found in lookup table it will return 1 or 0 . If not found what it will return below line? to return 0 in case , given key not available in lookup table , what I need to write?

Set isallowed = ##CLASS(Ens.Util.FunctionSet).Lookup("AllowLT", pRequest.GetValueAt("OBR:21"))

if isallowed=1

{

}

else

{

}