diff --git a/arrow.qmd b/arrow.qmd
index 30666cf..36f3e21 100644
--- a/arrow.qmd
+++ b/arrow.qmd
@@ -76,13 +76,15 @@ This means we want to avoid `read_csv()` and instead use the `arrow::open_datase
 ```{r open-dataset}
 seattle_csv <- open_dataset(
   sources = "data/seattle-library-checkouts.csv", 
+  col_types = schema(ISBN = string()),
   format = "csv"
 )
 ```
 
 What happens when this code is run?
 `open_dataset()` will scan a few thousand rows to figure out the structure of the dataset.
-Then it records what it's found and stops; it will only read further rows as you specifically request them.
+The `ISBN` column contains blank values for the first 80,000 rows, so we have to specify the column type to help arrow work out the data structure.
+Once the data has been scanned by `open_dataset()`, it records what it's found and stops; it will only read further rows as you specifically request them.
 This metadata is what we see if we print `seattle_csv`:
 
 ```{r}