I'm trying web scrape using R and have encountered some missing values when scraping gross revenue from the IMDB movies data. May I know how could I automatically insert NA if the movie's gross revenue is unknown?

```
webpage <- read_html('https://www.imdb.com/search/title/?count=100&release_date=2019,2019&title_type=feature')

gross <- html_nodes(webpage,'.ghost~ .text-muted+ span')
gross <- html_text(gross_data)

```