[Vala] invocation not supported in this context

[Nor Jaidi Tuah <norjaidi tuah ubd edu bn>]

When using "debug" in vala, I got this error:
 invocation not supported in this context

It hasn't happened before, and I can't figure
out what I did differently to cause this message.
I am still using the same version of vala,
the same compiler options, the same C compiler, etc. 

I did modify my source code, but all my modifications 
are "innocent".

Now, instead of debug, I have to use Glib.print

Any sane explanation?

hand
Nor Jaidi Tuah