trouble generating .ssx from shp

Hi,
I tried to generate ssx file with
D:\usr\lib\openmap-4.0\share\rivers>java -classpath
D:\usr\lib\openmap-4.0\lib\openmap.jar com.bbn.openmap.layer.shape.SpatialIndex
-c cities.ssx cities.shp

and got :
*** ERROR ***
Shapefile SpatialIndex.createIndex: Unknown shape type: 256
*************

did you encounter the same problem ?

(See attached file: cities.shp)

Salutations/Best regards
Olivier PONS

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