[Vala] Using "is" keyword yeilds error in Valac 0.18.1

[Tal Hadad <tal_hd hotmail com>]

Using "is" keyword yeilds error in Valac 0.18.1.
For example,
....
if (e is Error)
....

Gives me this error:
error: macro "G_TYPE_CHECK_INSTANCE_TYPE" requires 2 arguments, but only 1 given

In 0.16, it works 100%

Can you fix this please?
Tal