unable to use coplot function with zoo package

Readers,

I have tried to use the zoo package to merge datasets and then use the
coplot function, but the graph is not fully created. Only the panel
data is shown. Command terminal output below, with csv files. What is
the meaning of the warning message? Can anyone help please?

rhelpatconference.jabber.org
r 251
mandriva 2008

library(chron)
library(zoo)
z1<-read.zoo("test1.csv",header=FALSE,sep=",",FUN=times)
z2<-read.zoo("test2.csv",header=FALSE,sep=",",FUN=times)
z3<-read.zoo("test3.csv",header=FALSE,sep=",",FUN=times)
z4<-(na.approx(merge(z1,z2,z3),time(z1)))
coplot(z4$z2~z4$z1|z4$z3)

Warning messages:
1: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)
2: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)
3: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)
4: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)
5: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)
6: Index vectors are of different classes: integer times in:
merge.zoo(e1, e2, all = FALSE, retclass = NULL)

z4$z1

09:50:01 09:50:30 09:50:44 09:51:00 09:51:27 09:51:30 09:52:00 09:52:10
  320.61   325.43   320.48   315.53   314.63   313.73   316.11   323.88
09:52:30 09:52:54
  331.65   328.48

z4$z2

09:50:01 09:50:30 09:50:44 09:51:00 09:51:27 09:51:30 09:52:00 09:52:10
404.7500 402.7450 400.7400 300.2450 199.7500 266.7867 333.8233 400.8600
09:52:30 09:52:54
402.8350 404.8100

z4$z3

09:50:01 09:50:30 09:50:44 09:51:00 09:51:27 09:51:30 09:52:00 09:52:10
 166.120  175.770  185.420  198.395  211.370  214.820  218.270  221.720
09:52:30 09:52:54
 228.370  235.020

test1.csv:
09:50:00,315.79
09:50:30,325.43
09:51:00,315.53
09:51:30,313.73
09:52:00,316.11
09:52:30,331.65
09:53:00,325.31
09:53:30,334.33
09:54:00,334.54
09:54:30,336.55
09:55:00,339.34
09:55:30,341.93
test2.csv:
09:50:01,404.75
09:50:44,400.74
09:51:27,199.75
09:52:10,400.86
09:52:54,404.81
09:53:41,404.36
09:54:20,406.11
09:55:03,408.85
test3.csv:
09:50:01,166.12
09:50:44,185.42
09:51:27,211.37
09:52:10,221.72
09:52:54,235.02
09:53:41,262.85
09:54:20,306.64
09:55:03,346.3