R help - Sep 2011 - Trouble with performing post hoc analysis Tukey for lme model using ghlt

Hi, 

I am new to R and I am trying to perform a post hoc tukey test using the
multcomp package's ghlt for a lme model. My first attempt at doing so gave
me an output, HOWEVER I have tried to do this again, it keeps coming up with
the error:
 
(Error in contrMat(table(mf[[nm]]), type = types[pm]) : 
  less than two groups)

My model is looking at effect of incubation temperature (3 groups) on PHA
with hen as a my random factor. Temperature treatment has been made
factorial (Tempfact).

my code is as follows:

library(multcomp)
> PHA <- lme(DiffPHA48 ~ Tempfact, random= ~1|Hen, na.action=na.omit)
> summary(glht(PHA, linfct=mcp(Tempfact = "Tukey"))) 
Error in contrMat(table(mf[[nm]]), type = types[pm]) : 
  less than two groups

I am confused why it would work the first time and now everyother time it
comes up with this error. Can anyone please help me??

Thanks!

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