diff --git a/scraping/fetch.R b/scraping/fetch.R
index c9b416e..9faefe8 100644
--- a/scraping/fetch.R
+++ b/scraping/fetch.R
@@ -16,6 +16,7 @@ download_protocol <- function(path, name) {
 }
 
 fetch_batch <- function(offset) {
+    stopifnot("Offset must be numeric" = is.numeric(offset))
     mk_url(offset) %>%
         read_html() %>%
         as.character() %>%