WARNING: unknown action: calc

I'm adding an action to my xml:

<action name="some_action">
copy(current_fov, view.fov);
copy(ref_fov, hotspot[some_hotspot].rz);
calc(fov_delta, current_fov - ref_fov);
trace(get(fov_delta));
</action>

and, when I call the action, I get this error message:

"WARNING: unknown action: calc"

the calculation is not performed, and the trace action outputs "null"

using krpano html5 viewer, krpano 1.18.5 (build 2015-08-04)

1.19.pr7 is working.

Thanks!