using diff() in a program

[hpfx]

Hi,
I made a program that need to derivate a function, I don't understand why it does not work.
when I'm in home, I can type diff(3*X+1) I get the expected result.
But when I do the same in a program, I got error.

I tried all kind of possible syntax to make it work.
here is a simple example :

Code:

EXPORT ABCD()
BEGIN
'3*X+1'▶F9;
PRINT(diff(F9));
END;

as you can see, I use F9 to see if expression is correctly entered,
but I get this : "program(X,0,3)" instead.

is it a bug ???

FW version 10077