If/else condition in dplyr 0.7 function

I'd like to make a simple if/else condition in a dplyr function. I've looked at some helpful posts (e.g., How to parametrize function calls in dplyr 0.7?), but am still running into trouble.

Below is a toy example that works when I call the function without the grouping variable. The function then fails with the grouping variable.

# example dataset
test <- tibble(
  A = c(1:5,1:5),
  B = c(1,2,1,2,3,3,3,3,3,3),
  C = c(1,1,1,1,2,3,4,5,4,3)
)

# begin function, set default for group var to NULL.
prop_tab <- function(df, column, group = NULL) {

  col_name <- enquo(column)
  group_name <- enquo(group)

  # if group_by var is NOT null, then...
  if(!is.null(group)) {
      temp <- df %>%
        select(!!col_name, !!group_name) %>% 
        group_by(!!group_name) %>% 
        summarise(Percentages = 100 * length(!!col_name) / nrow(df))

  } else {
  # if group_by var is null, then...
      temp <- df %>%
        select(!!col_name) %>% 
        group_by(col_name = !!col_name) %>% 
        summarise(Percentages = 100 * length(!!col_name) / nrow(df)) 

  }

  temp
}

test %>% prop_tab(column = C)  # works

test %>% prop_tab(column = A, group = B)  # fails
# Error in prop_tab(., column = A, group = B) : object 'B' not found