diff --git a/readwise/api.py b/readwise/api.py
index 0f2a599..1b6d1e9 100644
--- a/readwise/api.py
+++ b/readwise/api.py
@@ -234,7 +234,9 @@ def get_books(
                     last_highlight_at=datetime.fromisoformat(book['last_highlight_at'])
                     if book['last_highlight_at']
                     else None,
-                    updated=datetime.fromisoformat(book['updated']),
+                    updated=datetime.fromisoformat(book['updated'])
+                    if book['updated']
+                    else None,
                     cover_image_url=book['cover_image_url'],
                     highlights_url=book['highlights_url'],
                     source_url=book['source_url'],
@@ -270,7 +272,9 @@ def get_book_highlights(
                     location_type=highlight['location_type'],
                     url=highlight['url'],
                     color=highlight['color'],
-                    updated=datetime.fromisoformat(highlight['updated']),
+                    updated=datetime.fromisoformat(highlight['updated'])
+                    if highlight['updated']
+                    else None,
                     book_id=highlight['book_id'],
                     tags=[
                         ReadwiseTag(id=tag['id'], name=tag['name'])
diff --git a/readwise/models.py b/readwise/models.py
index 3d59ab7..e1b952a 100644
--- a/readwise/models.py
+++ b/readwise/models.py
@@ -44,7 +44,7 @@ class ReadwiseBook:
     source: str
     num_highlights: int
     last_highlight_at: datetime | None
-    updated: datetime
+    updated: datetime | None
     cover_image_url: str
     highlights_url: str
     source_url: str
@@ -78,6 +78,6 @@ class ReadwiseHighlight:
     location_type: str
     url: str | None
     color: str
-    updated: datetime
+    updated: datetime | None
     book_id: str
     tags: list[ReadwiseTag]