I need to define a sensor for an element output (FLVEL2) to be retrieved in the UAMP user sub.

Even if from the manual it seems it is possible to define a sensor for a scalar quantity of an element set containing 1 single member, when I run the analysis the message 

OUTPUT REQUEST FLVEL2 CANNOT BE USED TO DEFINE A SENSOR

appears in the .dat file during preprocessing.

I have tried with other scalar element output variables (i.e. S11, POR...), but the same message appears.

Here there are the .inp file lines:

*elset,elset=el_ref

1

**********************************

*output,history, SENSOR, NAME=el_FLVEL2

*elementoutput,elset=el_ref

FLVEL2

Is there something I am missing?

Thank you very much