[R] linear mixed model using lmer

[array chip]

Dear all, I have this simple dataset to measure the yeild of a crop collected in 2 batches (attached). when I ran a simple inear mixed model using lmer to estimate within-batch and between-batch variability, the between-batch variability is 0. The run showed that data is singular. Does anyone know why the data is singular and what's the reason for 0 variability? is it because the dataset only has 2 batches?
> daty<-read.table("datx.txt",sep='\t',header=T,row.names=NULL)
> library(lme4)> lmer(yield~1+(1|batch),daty)
boundary (singular) fit: see ?isSingular
Linear mixed model fit by REML ['lmerMod']
Formula: yield ~ 1 + (1 | batch)
   Data: daty
REML criterion at convergence: 115.6358
Random effects:
 Groups   Name        Std.Dev.
 batch    (Intercept) 0.000   
 Residual             2.789   
Number of obs: 24, groups:  batch, 2
Fixed Effects:
(Intercept)  
      5.788  

Thanks!
John
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