[vtkusers] Eorrr:No data to generate normals for!

[Tracy]

Dear All, 

Does anybody know the reason to cause error message like 
"ERROR: In \Program Files\vtk42\VTK-4.2-LatestRelease\Vtk\Graphics\vtkPolyDataNormals.cxx, line 88
vtkPolyDataNormals (09C53780): No data to generate normals for!"? 

Actually what I do is following the example from "VTK\Example\Visual...\TCL\ExtractUgGrid.tcl with my own source file.
The format of my source file is exactly same as the original file "..\data\blow.vtk" except the specific data.
I do change the related parameters from the code file like SetScalarsName(), setVectorsName(), setCellMinimum() and SetCellMaximum().
But it doesn't  work well (I work it with C++.net).

Anybody can help me?

Thanks In advance.

Tracy
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