diff --git a/khal/khalendar/event.py b/khal/khalendar/event.py
index 7c3a0bbf2..d6674383d 100644
--- a/khal/khalendar/event.py
+++ b/khal/khalendar/event.py
@@ -524,8 +524,9 @@ def categories(self) -> str:
 
     def update_categories(self, categories: List[str]) -> None:
         assert isinstance(categories, list)
+        categories = [c.strip().lower() for c in categories if c != ""]
         self._vevents[self.ref].pop('CATEGORIES', False)
-        if categories:
+        if len(categories) > 0:
             self._vevents[self.ref].add('CATEGORIES', categories)
 
     @property