help with the nested anova formulas

 I am modeling grade as a function of membership in
 various cohorts.  There
are four "cohorts".  (NONE, ISE07,ISE08,ISE09) and two times of cohorts
coded as ISE = TRUE (ISE0#) or FALSE (NONE).  There is clear co-linearity
but that is to be expected.

 running the following code

 CutOff <-0
 fit.base <- lme(fixed= zGrade ~ Rep + COHORT/ISE + P7APrior + Female +
White + HSGPA + MATH + AP_TOTAL + Years + EOP + Course,
                 random= ~1|SID,
                 data = share[share$GRADE >= CutOff,])

 I get the following error

 Error in MEEM(object, conLin, control$niterEM) :
   Singularity in backsolve at level 0, block 1

 but if I take out the /ISE I get no error, simmilarly if I take out the
COHORT/.

 I want to test for the effects of the different cohorts within the ISE
subset and across ISE & NONE

I can send the data (the whole is too large) if you wish.