diff --git a/src/Handler/Utils/Exam.hs b/src/Handler/Utils/Exam.hs
index 211f27e4a..872d63c35 100644
--- a/src/Handler/Utils/Exam.hs
+++ b/src/Handler/Utils/Exam.hs
@@ -311,11 +311,14 @@ examAutoOccurrence (hash -> seed) rule ExamAutoOccurrenceConfig{..} occurrences
 
     occurrences' :: Map ExamOccurrenceId Natural
     -- ^ reduce room capacity for every pre-assigned user by 1
-    occurrences' = foldl' (flip $ Map.adjust predOrZero) occurrences $ Map.mapMaybe snd users
+    occurrences' = foldl' (flip $ Map.update predToPositive) occurrences $ Map.mapMaybe snd users
+    -- FIXME what about capacity-0 in occurrences?
+    -- what if the first word is too big for the first room?
       where
-        predOrZero :: Natural -> Natural
-        predOrZero 0 = 0
-        predOrZero n = pred n
+        predToPositive :: Natural -> Maybe Natural
+        predToPositive 0 = Nothing
+        predToPositive 1 = Nothing
+        predToPositive n = Just $ pred n
 
     occurrences'' :: [(ExamOccurrenceId, Natural)]
     -- ^ Minimise number of occurrences used