Error in ab() <- reactive(join(a(), b())) :  invalid (NULL) left side of assignment

a <- reactive({ MyData %>%
    filter( Primary_family == input$selected_product & Metric_name == input$selected_metric) %>%
    group_by(Month_considered) %>%
    mutate(`pct` = as.numeric(as.character(Metric_Value))*100 ) %>%
    select(Month_considered,`pct`) })

  b <-reactive({ MyData %>%
    filter(Tab == input$selected_class & Primary_family == input$selected_product) %>%
    group_by(Month_considered) %>%
    mutate(`ATC_Count` = Metric_Value) %>%
    select(Month_considered,`ATC_Count`) })

  ab() <-reactive(join(a(),b()))