lme model: Error in MEEM

Hi,
We have data of two groups of subjects: 32 elderly, 14 young adults. for
each subject we have 15 observations, each observation consisting of a
reaction-time measure (RT) and an activation maesure (betadlpcv). 
since we want to analyze the influence of (age-)group and RT on the
activation, we call: 

lme(betadlpcv ~ RT*group, data=our.data, random=~ RT |subject)

this yields:
Error in MEEM(object, conLin, control$niterEM) :
        Singularity in backsolve at level 0, block 1
In addition: Warning message:
Fewer observations than random effects in all level 1 groups in:
lme.formula(betadlpcv ~ RT * group, data = patrizia.data, random = ~RT |

what's the problem here?

thanks for your kind help
christoph

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