panel.abline on "lattice plot in Shiny" got error

[wxin]

I don't know very well about lattice but I know here the openair::timePlot function call lattice.xyplot to plot time series of pollutant concentrations.

I'd like to add ablines accordingly onto different panels. I successfully did it locally. But got error when use it in
Shiny.

Here are Shiny app code and screenshot.
The first abline using direct h=15 works, but second one get error

ps: 'mydata' is not MY data, it is included in Openair package

library(openair)
library(shiny)
library(latticeExtra)

ui <- fluidPage(
    plotOutput("timePlot")
)

server <- function(input, output) {
    output$timePlot <- renderPlot({
        df <- selectByDate(mydata, year = 2003, month = 3)
        timePlot(df, pollutant = c("so2", "co","pm25"),              
                y.relation = "free")
        ref <- c(15, 1, 40)
        trellis.last.object() +
            layer(panel.abline(h = 15, lty = 5), rows = 1) +
            layer(panel.abline(h = ref[2], lty = 5), rows = 2) 
        })
}

shinyApp(ui = ui, server = server)

Any help, Thanks!