diff --git a/scoping_review.qmd b/scoping_review.qmd
index f953e9e..3fb64d7 100644
--- a/scoping_review.qmd
+++ b/scoping_review.qmd
@@ -565,7 +565,9 @@ analysing the main findings per policy area, as well as underscore individual st
 #| fig-cap: Available studies by primary type of intervention
 
 by_intervention = (
-    bib_df.groupby(["author", "year", "title", "design", "method", "representativeness", "citation"])
+    bib_df
+    .fillna("")
+    .groupby(["author", "year", "title", "design", "method", "representativeness", "citation"])
     .agg(
         {
             "intervention": lambda _col: "; ".join(_col),