diff --git a/src/util.py b/src/util.py
index cc454b6..4fe77b7 100644
--- a/src/util.py
+++ b/src/util.py
@@ -19,7 +19,7 @@ def dataframe_to_urlmap(df: pd.DataFrame) -> dict[httpx.URL, set[httpx.URL]]:
     :return: A dictionary where the key is a source URL, and the value is a set of target URLs.
     """
     urlmap = {}
-    for source, target in zip(df["source"], df["target"], strict=True):
+    for source, target in zip(df.get("source", pd.Series()), df.get("target", pd.Series()), strict=True):
         source_url = httpx.URL(source)
         target_url = httpx.URL(target)
         if source_url not in urlmap: