Custom fix does not compute scalar?

Dear all,

I’ve written a simple fix just for learning the framework. In my header file I defined a method “double compute_scalar()” and in my corresponding *.cpp-file
I have

double FixMYFIX::compute_scalar()
{
return 10.0;
}

The compilation works fine, no errors, no warnings.
However, when I tell lammps to print the result of my fix to the thermo-output:

fix 5 testgroup myfix param1 param2 …

thermo_style custom step f_5 …

lammps always complains

“ERROR: Thermo fix does not compute scalar”

Have I missunderstood something?

best regards,
frank.