Why can not VPC be generated?

[Johnny0514]

Dear everyone:
I executed the NPDE validation command, and the plot of NPDE can be generated successfully, but the plot of VPC can not be generated, as displayed nothing.
The commands I used were as follows:
run6 <- PM_load(6)
valid <- run6$validate(limits = c(0, 3))
plot(valid, type = "npde", log = T, outeq = 1)
plot(valid, type = "vpc", log = T, outeq = 1)
The plot of NPDE can be generated:

But there was nothing for VPC.
I also tried the example data and commands:
library(PmetricsData)
valid <- NPex$validate(limits = c(0, 3))
plot(valid, type = "npde", log = T, outeq = 1)
plot(valid, type = "vpc", log = T, outeq = 1)
The results were the same; the plot of NPDE can be generated, except for VPC.
How can I generate the VPC?
Look forward to your comment!
Kind regards!