[R-SIG-Mac] Error in GEE models

[david dav]

Hi,
I would like to fit a GLM model with GEE on clustered data.
I tried to use gee in the GEE package on a twin data set. All cluster
are of size 2. I removed the missing data first.

> gee(Y ~ X1 + X2,
+     id = IDENTIF,  family = binomial, corstr = "unstructured",
+     data =GP)

I get these results :
Beginning Cgee S-function, @(#) geeformula.q 4.13 98/01/27
running glm to get initial regression estimate
(Intercept)         X1M  X2Sans AMP      X2Stim
 -2.4791229   0.4250848  -0.1310666  -0.2151235
Error in gee(Y ~ X1 + X2, id = IDENTIF, family = binomial, corstr =
"unstructured",  :
  NA/NaN/Inf dans un appel `a une fonction externe (argument 2)#in a
call to an external function would be the translation
De plus : Warning message:
In gee(Y ~ X1 + X2, id = IDENTIF, family = binomial, corstr = "unstructured",  :
  NAs introduits lors de la conversion automatique

geepack does not work any better when the regression gives results
with SAS version 8 on the same dataset (results are pretty close from
the results of a naive logistic regression).
I tried to change the explanatory variables (X1 alone, X2 alone, other
possible explanatory vairables : no result). I do not find where the
error comes from.

Thank you for your help.
David
Using R2.7.2 and gee version 4.13-13

PS Naive logistic regression give similar results on R and SAS in my case