Spatio-temporal variogram error

hi, am doing a spatio-temporal variogram and am getting this error below
that i have no response variable in my data set even though it is there. my
data am calling gdata and the response variable is pfpr. i have tried the
code below with no access. the summary of my data is as follows:

summary(gdata)

Object of class STFDF
  with Dimensions (s, t, attr): (438, 21, 1)
[[Spatial:]]
Object of class SpatialPoints
Coordinates:
          min      max
Long -16.718 -13.8500
Lat   13.176  13.7915
Is projected: NA
proj4string : [NA]
Number of points: 438
[[Temporal:]]
      Index              timeIndex
  Min.   :1990-01-01   Min.   : 1
  1st Qu.:1994-12-31   1st Qu.: 6
  Median :1999-12-30   Median :11
  Mean   :1999-12-30   Mean   :11
  3rd Qu.:2004-12-28   3rd Qu.:16
  Max.   :2009-12-27   Max.   :21
[[Data attributes:]]
    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.    NA's
     0.0     1.0     4.0    10.1    13.0   330.0    8640

s=variogram(gdata at data~1, gdata)

   |

   |
                                |   0%Error in .subset2(x, i, exact = exact)
: no such index at level 1

s=variogram(pfpr~1, gdata)

   |

   |
                                |   0%Error in gstat.formula(d$formula,
d$data) :
   no response variable present in formula
In addition: Warning messages:
1: In min(x) : no non-missing arguments to min; returning Inf
2: In max(x) : no non-missing arguments to max; returning -Inf
3: In min(x) : no non-missing arguments to min; returning Inf
4: In max(x) : no non-missing arguments to max; returning -Inf

can anyone help?
J

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