nesting in CoxPH with survival package

Hello all,
I am trying to understand how to specify nested factors when using
coxph(), and if it is appropriate to nest these factors in my
situation.
In the simplest form, I am testing two different temperatures, with
each temperature being performed twice in different experimental
periods (e.g. Temp5 performed in Period A and C, Temp4 performed in
Period B and D)
I am trying to see if survival time is affected by the treatment
temperature.  To do this I am using temperature and experimental
period nested within temperature as factors.

LOEtempmod.5days=coxph(LOE.stable.5days~Temp+Temp/Period,data=goodstable)

Warning message:
In coxph(LOE.stable.5days ~ Temp + Temp/Period,  :
  X matrix deemed to be singular; variable 2 5 6 7

1. Is this an appropriate way of nesting?
2. Can this error message be ignored?

Any help would be much appreciated, thank you in advance.

Katie