Question re DF for testing covariate in lme() (W2k)

[KINLEY_ROBERT@LILLY.COM]

I'm puzzled by the fact that lme() reports  1 DF  for the denominator in the t-test , and also for the t-value used in calculating the confidence interval, for the slope term in the model described below .  

... if anyone can point me in the right direction I'd be very grateful.

Details :- 

I have the dataset called 'TF' :-

  Strength     type AnalDay    Assay StorageDays AssayOcc 
1        4 FinalSol       0  40.4355           0        1
2        4 FinalSol       4  41.0857           4        2
3        4 FinalSol       7  40.8318           7        3
4       10 FinalSol       0 103.6812           0        1
5       10 FinalSol       4 104.5013           4        2
6       10 FinalSol       7 104.1406           7        3
7       20 FinalSol       0 195.6095           0        1
8       20 FinalSol       4 196.6439           4        2
9       20 FinalSol       7 196.1706           7        3

and I'm trying to fit the model   Assay = Strength + AssayOcc + StorageDays 

where 
Strength is a fixed effect factor, 
StorageDays is a covariate ... a fixed slope effect , assumed common to all 3 strengths
AssayOcc is a random effect for the variability between assay occasions

[ Perhaps the model itself is inappropriate ..? ]

> TF.lme <- lme(fixed = Assay ~ Strength + StorageDays, data = "" random =  ~ 1 | AssayOcc)
> summary(TF.lme)

Linear mixed-effects model fit by REML
 Data: TF 
       AIC      BIC    logLik 
  18.22203 15.87866 -3.111015

Random effects:
 Formula:  ~ 1 | AssayOcc
        (Intercept)   Residual 
StdDev:   0.4563897 0.09662428

Fixed effects: Assay ~ Strength + StorageDays 
               Value Std.Error DF  t-value p-value 
(Intercept) 113.4024 0.4309204  4  263.163  <.0001
  Strength1  31.6617 0.0394467  4  802.645  <.0001
  Strength2  41.2318 0.0227746  4 1810.431  <.0001
StorageDays   0.0751 0.0925766  1    0.811  0.5661
                                   --

Standardized Within-Group Residuals:
       Min        Q1           Med         Q3      Max 
 -1.019119 -0.107893 -0.0007243514 0.08046544 1.127012

Number of Observations: 9
Number of Groups: 3 

 > intervals(TF.lme)

Approximate 95% confidence intervals

 Fixed effects:
                  lower         est.      upper 
(Intercept)  112.205987 113.40241396 114.598841
  Strength1   31.552162  31.66168333  31.771205
  Strength2   41.168540  41.23177222  41.295005
StorageDays   -1.101194   0.07510225   1.251399

 Random Effects:
  Level: AssayOcc 
                    lower      est.    upper 
sd((Intercept)) 0.1115825 0.4563897 1.866705

 Within-group standard error:
      lower       est.    upper 
 0.04832077 0.09662428 0.193214


 If anyone can help I'd be most grateful

                                thanks                Bob