zoo: variable gets modified at making zoo object

I'm doing:

alyL32007z <- zoo(alyL32007,alyL32007$time)
range(time(alyL32007z))

[1] "2007-01-01 00:00:00 UTC" "2007-12-31 23:30:00 UTC"

But then, while the original variable is:

summary(alyL32007$NEE_st)

? Min. 1st Qu. ?Median ? ?Mean 3rd Qu. ? ?Max. ? ?NA's
-15.340 ?-1.615 ?-0.054 ?-0.814 ? 0.750 ? 8.965 ? 11124

the variable within the zoo object is different:

summary(alyL32007z$NEE_st)

? ? Index ? ? ? ? ? ? ? ? ? ? alyL32007z$NEE_st
?Min. ? :2007-01-01 00:00:00 ? ? 0.335: ? ?7
?1st Qu.:2007-04-02 05:52:30 ? ? 0.582: ? ?7
?Median :2007-07-02 11:45:00 ? ? 0.611: ? ?7
?Mean ? :2007-07-02 11:45:00 ? ? 0.063: ? ?6
?3rd Qu.:2007-10-01 17:37:30 ? ? 0.069: ? ?6
?Max. ? :2007-12-31 23:30:00 ? (Other): 6363
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? NA's ? :11124

and I ?get an error at plotting:

plot(alyL32007z$NEE_st)

Error in plot.window(...) : invalid 'ylim' value

Any help appreciated,

Thanks
Agus
--
Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
e-mail Agustin.Lobo at ictja.csic.es
https://sites.google.com/site/aloboaleu/