Esri Shape files causing unhandled exception error

I am using a program called flo-2d to do some flood modeling. The
ouput from flo-2d are esri shape files. I am importing them into
LDDv2006 using the import command in the maps menu. Most of the shape
files import fine, but I am having issues with importing area shape
files as mpolygons. The shape files import normally, and the file will
save. Upon trying to re-load the drawing, I get an unhandled expeption
error.
Has anyone else encountered this problem and if so, how did you work
around it?
Thanks,
C. Alan