Columns system.dbnull

The example would be like this:
<<<query analizer>>>
sp1 param1,param2
--shows
col1 col2 col3 col4
---- ---- ---- ----
1 1 2 3
11 2 2 4
2 3 0 9


<<<vb.NET>>>
dbl.addparam(param1)
dbl.addparam(param2)
ds = dbl.runproc("SP1")
ds.tables(0).rows(0)(3) ' or ds.tables(0).rows(0)("col4")
===SYSTEM.DBNULL???
'instead data last 3 columns have just system.dbnull...


Thanks in advance for some advice post...
Pd: could this behavior( bug) be caused by my sp??
the most strange i did inside it is add some registers in a temporary
#table created by a select clause..