problem for specifiying refrence cell
 

hello Foamers,
i am modeling a simple channel flow using the interFoam just like the waterchannel tutorial provided in the openfoam..so when i chnged the property of walls to empty for front and back walls...it gives the following error as i am a newbee i cant understand this...
FOAM FATAL IO ERROR:
Unable to set reference cell for field p
Please supply either pRefCell or pRefPoint


file: /home/user/OpenFOAM/user-2.2.0/run/squareBump12/system/fvSolution.PIMPLE from line 68 to line 74.

From function void Foam::setRefCell
(
const volScalarField&,
const volScalarField&,
const dictionary&,
label& scalar&,
bool
)
in file cfdTools/general/findRefCell/findRefCell.C at line 125.

please help..:confused: