semantic error: unresolved target-symbol expression: identifier '$filename'

Question

I'm trying to follow some simple examples like this one here:

probe syscall.open.return {
 filename = user_string($filename)
 if (execname() != "yakuake"){
 printf("%s(%d) open (%s)\n", execname(),pid(),filename)
 }
}

That I've found in the following link:

https://education.dellemc.com/content/dam/dell-emc/documents/en-us/2014KS_Ahmed-SystemTap_for_Linux_Platforms.pdf

However, I'm always getting the following semantic error:

semantic error: while processing probe nd_syscall.open.return from: syscall.open.return from: syscall.open.return

semantic error: unresolved target-symbol expression: identifier '$filename' at test.stp:2:35
        source:     filename = user_string(@entry($filename))

Do someone knows what is happening?

I'm using systemtap for Systemtap translator/driver (version 3.1/0.168, rpm 3.1-3.el7), on Red Hat Enterprise Linux Workstation release 7.4

I'm saving the code to a file called "test.stp" and then run the code with:

sudo stap test.stp