cooper mesh error
 

I am using the cooper scheme meshing a volume. The two sources faces of the volume consist of several samll faces.When i try cooper shceme,the error comes up.

Error:Assertion failed in D\home\devuser\Fi2.4\gambit2.4\include\Dllist.hpp; Line 418:my index>=0&&mu index<itemCount while meshing volume 1

Does anyone can tell me what is the error meaning?