[R-sig-ME] Error message with lme function

[FMH]

Dear All,

I have a set of data which consist of 261 groups with 11 temperature values in each group. This temperature is recorded across 11 different depths of the sea. The variables involve in the R script are:

############################
#group : 1-261
#dp: 11 different depths for each group
#sub1: Data set 1
#set: Data set 2
############################

I tried to fit a linear mixed model with fixed efffect on slope and intercept, and random effect on slope and intercept too with lme function, but sadly the program gave an error message. 

#################################################
dp<- rep(rev(seq(1,51, by = 5)),261)
group <- rep(1:261, each = 11)
set <- data.frame(sub1, dp, group)
(lm.lme2 <- lme(Temp ~ dp| group, data = set, random = ~ dp| group))

Error in MEEM(object, conLin, control$niterEM) : 
 Singularity in backsolve at level 0, block 1
#################################################

Could someone please advice me the meaning of this error and the way to sort it out?

Thank you
Fir