diff --git a/src/spanreed/plugins/timekiller.py b/src/spanreed/plugins/timekiller.py
index 7abc171..4cf50d0 100644
--- a/src/spanreed/plugins/timekiller.py
+++ b/src/spanreed/plugins/timekiller.py
@@ -67,9 +67,12 @@ async def get_available_time_killers(
             user, self.LAST_ASKED_BOOKS_KEY
         )
         if last_asked_str is not None:
-            last_asked = datetime.datetime.fromisoformat(
-                last_asked_str
-            )
+            try:
+                last_asked = datetime.datetime.fromisoformat(
+                    last_asked_str
+                )
+            except TypeError:
+                raise last_asked_str
         if datetime.datetime.now() - last_asked > datetime.timedelta(days=3):
             timekillers["Books"] = self.prompt_for_currently_reading_books