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?