Bug in shiny's renderUI function?

server.R

require(shiny)

shinyServer(function(input, output) {
  
  val <- reactive({
    if(input$change>0) {
      # Option 1
       'abc'
      #       # Option 2
      #       isolate({
      #         paste('abc',input$txt,"")
      #       })
    } else {
      ''
    }
  })
  
  output$textbox <- renderUI({
    textInput("txt","Text",val())
  })
  
})

ui.R

require(shiny)
require(shinyIncubator)

shinyUI(pageWithSidebar(
  headerPanel('Test'),
  
  sidebarPanel(
    uiOutput("textbox"),
    actionButton("change", "Change")
  ),
  
  mainPanel(
    
  )
))