Abaqus Explicit Trouble to define a Fluid Cavity

[max62niers]

Hello Everybody,

I am currently trying to define a Fluid Cavity with a gas inside.
I am relatively new with Abaqus and working in the keywords gives me problems.
I guess I did everything to define this cavity (thanks to the Keyword Reference Manual and to the Analysis User Manual) : I put the *FLUID CAVITY with all the required parameters, *FLUID BEHAVIOR with *CAPACITY and *MOLECULAR WEIGHT..

but I still have this recurrent error saying:

"Fluid Cavity defined by reference node number **** has zero or negative mass"...

I can't find what the trouble,
I will be very please if one of you could suggest me a solution,

Thanks in advance!

Maxime