using else if in a R shiny app

I have a simple sample size calculator shiny app im trying to build that I cant seem to figure out.

the inputs consist of 1 numerical input and a series of radio buttons. Im trying to have a different calculation carried dependent on which radiop button is selected

 ui <- shinyUI(fluidPage(
  titlePanel("alpha"),

  sidebarPanel(
    numericInput(
      "expn",
      "Please enter total number of reports received",
      1,
      min = 0,
      max = 1000000
    ),
    radioButtons(
      inputId = "'QC_Type'",
      label = "QC Type",
      choices = c(
        "Overall" = "a",
        "Solicited" ="b",
        "Spontaneous" = "c",
        "Clinical Trial"="d",
        "Literature"= "e"
      ) 
    )
  ),
  mainPanel (
    p ("Your required sample size is"),
    textOutput("results"))
))

server <- function (input, output) {
  A1 <- (1.96*1.96)*(0.3)*(1-0.3)/(0.05*0.05)
  B1 <- (1.96*1.96)*(0.32)*(1-0.32)/(0.05*0.05)
  C1 <-  (1.96*1.96)*(0.35)*(1-0.35)/(0.05*0.05)
  D1 <-  (1.96*1.96)*(0.26)*(1-0.26)/(0.05*0.05)
  E1 <-  (1.96*1.96)*(0.36)*(1-0.36)/(0.05*0.05)
  options(digits=2)
if (input$QC_Type=="a") {
output$results <- renderText({
as.numeric( (input$expn)*(A1)/(input$expn+(A1)+1))
})
} else if (input$QC_Type=="b") {
output$results <- renderText({
as.numeric( (input$expn)*(B1)/(input$expn+(B1)+1))
})
} else if (input$QC_Type=="c") {
output$results <- renderText({
as.numeric( (input$expn)*(C1)/(input$expn+(C1)+1))
})
} else if (input$QC_Type=="d") {
output$results <- renderText({
as.numeric( (input$expn)*(D1)/(input$expn+(D1)+1))
  })
} else if (input$QC_Type=="e") {
  output$results <- renderText({
    as.numeric( (input$expn)*(E1)/(input$expn+(E1)+1))
  })
}
}








shinyApp(ui = ui, server = server)

While the "front end" elements are fine - I cant get the server side to work I get the error message

Error in .getReactiveEnvironment()$currentContext() : 

Operation not allowed without an active reactive context

Any advice or solutions would be massively appreciated

Cheers