R help - Jun 2009 - anova with missing data (incomplete design)

hi,


I have the following data:


x1		y1		y2		y3
1.3		grp1	a1		NA
2.4		grp1	a1		NA
3.7		grp1	a2		NA
1.9		grp1	a2		NA
2.2		grp2	NA		b1
3.5		grp2	NA		b1
7.7		grp2	NA		b2
6.0		grp2	NA		b2


how can I analyse this with the anova-function?

I get the error message
"error in `contrasts<-`(`*tmp*`, value = "contr.treatment") :
   contrasts can be applied only to factors with 2 or more levels"