diff --git a/test/Handler/Utils/ExamSpec.hs b/test/Handler/Utils/ExamSpec.hs
index 11be48ed3..2566ab76a 100644
--- a/test/Handler/Utils/ExamSpec.hs
+++ b/test/Handler/Utils/ExamSpec.hs
@@ -16,9 +16,6 @@ import qualified Data.CaseInsensitive as CI
 
 import Handler.Utils.Exam
 
--- TODO
--- use frequency instead of elements?
--- are these capacity values realistic?
 instance Arbitrary ExamOccurrence where
     arbitrary = ExamOccurrence
         <$> arbitrary   -- examOccurrenceExam
@@ -42,6 +39,9 @@ data Nudges = NoNudges | SmallNudges | LargeNudges
 uncurry3 :: (a -> b -> c -> d) -> (a, b, c) -> d
 uncurry3 f (a, b, c) = f a b c
 
+uncurry4 :: (a -> b -> c -> d -> e) -> (a, b, c, d) -> e
+uncurry4 f (a, b, c, d) = f a b c d
+
 -- | Kopie der User-Datenstruktur beschränkt auf interessante Felder (besser verständliche Show-Instanz)
 data UserProperties = UserProperties {pSurname :: Text, pMatrikelnummer :: Maybe Text}
     deriving (Show)
@@ -62,14 +62,12 @@ extractProperties User {userSurname, userMatrikelnummer} = UserProperties userSu
 spec :: Spec
 spec = do
     describe "examAutoOccurrence" $ do
-        {-
         describe "Surname" $ do
             let rule :: ExamOccurrenceRule
                 rule = ExamRoomSurname
             forM_ universeF $ \nudges -> describe (show nudges) $
                 forM_ universeF $ \preselection ->
                     prop (show preselection) $ propertyTest rule nudges preselection
-        -}
         describe "Matriculation" $ do
             let rule :: ExamOccurrenceRule
                 rule = ExamRoomMatriculation
@@ -92,7 +90,7 @@ spec = do
                                 in foldM (genNudge nudgeFrequency) Map.empty $ Map.keys occurrences
             let config :: ExamAutoOccurrenceConfig
                 config = def {eaocNudge}
-                result@(_maybeMapping, userMap) = examAutoOccurrence seed rule config occurrences users
+                (maybeMapping, userMap) = examAutoOccurrence seed rule config occurrences users
             pure $ ioProperty $ do
                 -- every (relevant) user got assigned a room
                 shouldBe (length userMap) (length users)
@@ -109,8 +107,12 @@ spec = do
                 let userProperties :: Map UserId (UserProperties, Maybe ExamOccurrenceId)
                     userProperties = Map.map (first extractProperties) users
                 shouldSatisfy (userProperties, occurrences, userMap) $ uncurry3 fitsInRooms
-                -- all users match the shown ranges
-                shouldSatisfy (rule, userProperties, result) $ uncurry3 showsCorrectRanges
+                case maybeMapping of
+                    -- all users match the shown ranges
+                    (Just occurrenceMapping)
+                        -> shouldSatisfy (rule, userProperties, occurrenceMapping, userMap) $ uncurry4 showsCorrectRanges
+                    -- is a nullResult justified?
+                    Nothing -> shouldSatisfy (rule, userProperties, occurrences) $ uncurry3 isNullResultJustified
         -- | generate users without any pre-assigned rooms
         genUsersWithOccurrences :: Preselection -> Gen (Map UserId (User, Maybe ExamOccurrenceId), Map ExamOccurrenceId Natural)
         genUsersWithOccurrences preselection = do
@@ -178,10 +180,10 @@ spec = do
         -- Users with a previously assigned room are checked if the assignment stays the same, regardless of the ranges.
         showsCorrectRanges :: ExamOccurrenceRule
                             -> Map UserId (UserProperties, Maybe ExamOccurrenceId)
-                            -> (Maybe (ExamOccurrenceMapping ExamOccurrenceId), Map UserId (Maybe ExamOccurrenceId))
+                            -> ExamOccurrenceMapping ExamOccurrenceId
+                            -> Map UserId (Maybe ExamOccurrenceId)
                             -> Bool
-        showsCorrectRanges _rule _userProperties (Nothing, _userMap) = False
-        showsCorrectRanges rule userProperties (Just (examOccurrenceMappingMapping -> mappingRanges), userMap)
+        showsCorrectRanges rule userProperties (examOccurrenceMappingMapping -> mappingRanges) userMap
             = all userFitsInRange $ Map.toAscList $ occurrenceMap userMap
             where
                 userFitsInRange :: (ExamOccurrenceId, [UserId]) -> Bool
@@ -204,3 +206,47 @@ spec = do
                                 fitsInRange ExamOccurrenceMappingSpecial {}
                                     = True      -- FIXME what is the meaning of special?
                         _otherwise -> False
+        -- | Is mapping impossible?
+        isNullResultJustified :: ExamOccurrenceRule
+                                -> Map UserId (UserProperties, Maybe ExamOccurrenceId)
+                                -> Map ExamOccurrenceId Natural -> Bool
+        isNullResultJustified rule userProperties occurrences
+            = noRelevantUsers rule userProperties || mappingImpossible rule userProperties occurrences
+        noRelevantUsers :: ExamOccurrenceRule -> Map UserId (UserProperties, Maybe ExamOccurrenceId) -> Bool
+        noRelevantUsers rule = null . Map.filter (isRelevantUser rule)
+        isRelevantUser :: ExamOccurrenceRule -> (UserProperties, Maybe ExamOccurrenceId) -> Bool
+        isRelevantUser _rule (_user, Just _assignedRoom) = False
+        isRelevantUser rule (UserProperties {pSurname, pMatrikelnummer}, Nothing) = case rule of
+            ExamRoomSurname -> not $ null pSurname
+            ExamRoomMatriculation -> maybe False (not . null) pMatrikelnummer
+            _rule -> False
+        mappingImpossible :: ExamOccurrenceRule -> Map UserId (UserProperties, Maybe ExamOccurrenceId) -> Map ExamOccurrenceId Natural -> Bool
+        mappingImpossible
+            rule
+            userProperties@(sort . map (ruleProperty rule . fst) . Map.elems . Map.filter (isRelevantUser rule) -> relevantUsers)
+            (map snd . Map.toList . adjustOccurrences userProperties -> occurrences') = go relevantUsers occurrences'
+            where
+                go :: [Maybe Text] -> [Natural] -> Bool
+                go [] _occurrences = False
+                go _remainingUsers [] = True
+                go remainingUsers (0:t) = go remainingUsers t
+                go remainingUsers@(h:_t) (firstOccurrence:laterOccurrences)
+                    | nextUsers <= firstOccurrence = go remainingUsers' $ firstOccurrence - nextUsers : laterOccurrences
+                    | otherwise = go remainingUsers laterOccurrences
+                    where
+                        (fromIntegral . length -> nextUsers, remainingUsers') = span (== h) remainingUsers
+        ruleProperty :: ExamOccurrenceRule -> UserProperties -> Maybe Text
+        ruleProperty rule = case rule of
+            ExamRoomSurname -> Just . pSurname
+            ExamRoomMatriculation -> pMatrikelnummer
+            _rule -> const Nothing
+        -- copied and adjusted from Hander.Utils.Exam
+        adjustOccurrences :: Map UserId (UserProperties, Maybe ExamOccurrenceId) -> Map ExamOccurrenceId Natural -> Map ExamOccurrenceId Natural
+        -- ^ reduce room capacity for every pre-assigned user by 1
+        adjustOccurrences userProperties occurrences = foldl' (flip $ Map.update predToPositive) occurrences $ Map.mapMaybe snd userProperties
+        -- FIXME what about capacity-0 in occurrences?
+        -- what if the first word is too big for the first room?
+        predToPositive :: Natural -> Maybe Natural
+        predToPositive 0 = Nothing
+        predToPositive 1 = Nothing
+        predToPositive n = Just $ pred n