diff --git a/book/lectures/143-observability-of-code-outputs.qmd b/book/lectures/143-observability-of-code-outputs.qmd
index da1afe5..eff2e81 100644
--- a/book/lectures/143-observability-of-code-outputs.qmd
+++ b/book/lectures/143-observability-of-code-outputs.qmd
@@ -45,6 +45,8 @@ cars_ggplot_scatter$mapping$x
 And so we could write some tests for a function that created a `ggplot2` object like so:
 
 ```{r}
+library(testthat)
+
 #' scatter2d
 #'
 #' A short-cut function for creating 2 dimensional scatterplots via ggplot2.