plm random effect: the estimated variance of the individual effect is negative

Hi,R 2.15.2 plm() function on Windows 7
I have a problem with the variance estimation in a random effect model
I used this formula to get my result, and it works:reg <- deltaF ~  
L1.deltaF + L2.deltaF + deltaCDS + L1.deltaCDS + L2.deltaCDS +  
L3.deltaCDS + deltaUS_Yields + deltaZ + L1.deltaZ + L2.deltaZfixed  
<- plm(reg, data=Mody_R, na.action = na.exclude,  
index=c("country_id", "datestata"), model="random")
but when i split my dataframe into several groups it gives me this  
error:"Error in swar(object, data, effect) :   the estimated  
variance of the individual effect is negative"
The code i used to split the dataframe is:SplitMody <- split(Mody_R,  
Mody_R$period)SplitFixed <- lapply(SplitMody, function(Mody_R)  
plm(reg, data=Mody_R, na.action = na.exclude, index=c("country_id",  
"datestata"), model="random"))
I want to point the all the code works perfectly in a within model,  
and I tried using all the random.method for the variance  
estimation.In addition I used the same model in Stata and it  
works... (the results are identical for a within model in R and Stata)
I'll be really gratefull if someone can help me.
Thanks,Matteo 		 	   		   		 	   		
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