[R] NA with lmList

[FMH]

Dear All,
  
I was trying to use the lmList function to get the lmList graphic but  have a 
problem creating the graphic.  My data has missing values and an error occurred  
as stated below. 



###################################################

library(nlme)

> a 
  schoolid spring score childid 
1 1 0 550 345 
2 1 1 568 345 
3 1 0 560 456 
4 1 1 NA 456 
5 2 0 540 32 
6 2 1 562 32 
7 2 0 579 34 
8 2 1 599 34 


(lmlis1 <- lmList(score ~ childid | spring, data=a, na.action=T))

Error in UseMethod("getGroups") : no applicable method for "getGroups"

#######################################################


Could anybody advice me the way to write the command correctly, which ignore the 
missing data?

Cheers
Fir