density /diversity of points

I've tried your method to determine the richness of landscapes on my area of
interest using european corine land cover.

I converted my polygon layer into a SpatialGridDataFrame then into a
SpatialPointsDataFrame, and finally into a ppp object using one column: the
type of soil occupation (CLC3_LIB).

But my problem is that marktable gives me zero values, although I should get
at least a "1" value for one of my columns...

Here is a sample code:

ppp[1:5]$marks

[1] Mer et oc?an
[2] V?g?tation clairsem?e
[3] Pelouses et p?turages naturels
[4] V?g?tation scl?rophylle (y.c. maquis et garrigue)
[5] V?g?tation scl?rophylle (y.c. maquis et garrigue)

marktable(ppp[1:10], R=50)

      mark
point A?roports Chantiers Cours et voie d'eau
    1          0         0                   0
    2          0         0                   0
    3          0         0                   0
    4          0         0                   0
    5          0         0                   0
    6          0         0                   0
    7          0         0                   0
    8          0         0                   0
    9          0         0                   0
    10         0         0                   0
      mark
point Equipements sportifs et de loisirs
    1                                   0
    2                                   0
    3                                   0
    4                                   0
    5                                   0
    6                                   0
    7                                   0
    8                                   0
    9                                   0
    10                                  0

I don't see where the problem is..

Any help would be greatly appreciated.