Creating new column for reactive dataframe in shiny/shiny dashboard R

within the server for my shinyApp, I created a dataframe based on the inputs. However, I want to add a new column that utilizes two of the columns of that dataframe.

server <- function(input, output, session) {
  l.out <- reactive({
    BatchGetSymbols(tickers = input$stock, 
                first.date = Sys.Date() - as.integer(input$length),
                last.date = Sys.Date())
  })
  stock_info <- reactive({
    l.out()$df.tickers
  })

  stock_info()$return <- reactive({
    rep(0, length(stock_info()$ref.date))
  })
  stock_info()$return <- reactive({
    for (i in 2:length(stock_info()$ref.date)){
      stock_info()$return[i] <- ((stock_info()$price.close[i] - 
stock_info()$price.close[i - 1]) / stock_info$price.close[i - 1])
    }
  })

I have tried it like this, and it works up until I try to create stock_info()$return, where I keep getting the error that NULL left assignment.
Any tips?