-- Bugfix #2868: NMesh.update(0,0,1) with derived mesh data would cause a

crash.  Hopefully the NMesh module will die a silent death before we
   discovere this doesn't really fix the problem :-)

source/blender/python/api2_2x/NMesh.c

@@ -1373,6 +1373,9 @@ static PyObject *NMesh_update( PyObject *self, PyObject *a, PyObject *kwd )
 			}
 			base = base->next;
 		}
+
+		/* recalculate the derived mesh before trying to use it */
+		makeDispListMesh (nmesh->object);
 		make_vertexcol();
 
 		countall();