diff --git a/hstream-kafka/HStream/Kafka/Group/Group.hs b/hstream-kafka/HStream/Kafka/Group/Group.hs
index a01e451ab..c7fde497e 100644
--- a/hstream-kafka/HStream/Kafka/Group/Group.hs
+++ b/hstream-kafka/HStream/Kafka/Group/Group.hs
@@ -916,11 +916,12 @@ fetchOffsets Group{..} reqTopic validateReqTopic = validateReqTopic reqTopic >>=
       , partitions = K.KaArray (Just $ (makeErrorPartition code) <$> partitions')
       }
   where
+    -- FIXME: hardcoded constants
     makeErrorPartition code idx =
       K.OffsetFetchResponsePartition
         { partitionIndex  = idx
         , committedOffset = -1
-        , metadata        = Nothing
+        , metadata        = Just ""
         , errorCode       = code
         }
 
diff --git a/hstream-kafka/HStream/Kafka/Group/GroupOffsetManager.hs b/hstream-kafka/HStream/Kafka/Group/GroupOffsetManager.hs
index 3f4161dc4..a59b48180 100644
--- a/hstream-kafka/HStream/Kafka/Group/GroupOffsetManager.hs
+++ b/hstream-kafka/HStream/Kafka/Group/GroupOffsetManager.hs
@@ -189,18 +189,19 @@ fetchOffsets GroupOffsetManager{..} topicName partitions = do
     , partitions = KaArray {unKaArray = Just res}
     }
  where
+   -- FIXME: hardcoded constants
    getOffset cache partitionIdx = do
      let key = mkTopicPartition topicName partitionIdx
       in case Map.lookup key cache of
            Just offset -> return $ OffsetFetchResponsePartition
              { committedOffset = offset
-             , metadata = Nothing
+             , metadata = Just ""
              , partitionIndex= partitionIdx
              , errorCode = K.NONE
              }
            Nothing -> return $ OffsetFetchResponsePartition
              { committedOffset = -1
-             , metadata = Nothing
+             , metadata = Just ""
              , partitionIndex= partitionIdx
              , errorCode = K.NONE
              -- TODO: check the error code here
@@ -214,9 +215,10 @@ fetchAllOffsets GroupOffsetManager{..} = do
   -- group offsets by TopicName
   cachedOffset <- Map.foldrWithKey foldF Map.empty <$> readIORef offsetsCache
   return . KaArray . Just . V.map makeTopic . V.fromList . Map.toList $ cachedOffset
+  -- FIXME: hardcoded constants
   where makePartition partition offset = OffsetFetchResponsePartition
                    { committedOffset = offset
-                   , metadata = Nothing
+                   , metadata = Just ""
                    , partitionIndex=partition
                    , errorCode = K.NONE
                    }
diff --git a/hstream-kafka/HStream/Kafka/Server/Handler/Offset.hs b/hstream-kafka/HStream/Kafka/Server/Handler/Offset.hs
index df4ae8387..f68186e0c 100644
--- a/hstream-kafka/HStream/Kafka/Server/Handler/Offset.hs
+++ b/hstream-kafka/HStream/Kafka/Server/Handler/Offset.hs
@@ -21,7 +21,8 @@ import           HStream.Kafka.Common.OffsetManager    (getLatestOffset,
                                                         getOffsetByTimestamp,
                                                         getOldestOffset)
 import           HStream.Kafka.Common.Resource
-import           HStream.Kafka.Common.Utils            (forKaArray, forKaArrayM)
+import           HStream.Kafka.Common.Utils            (forKaArray, forKaArrayM,
+                                                        vectorToKaArray)
 import qualified HStream.Kafka.Group.Group             as G
 import qualified HStream.Kafka.Group.GroupCoordinator  as GC
 import           HStream.Kafka.Server.Types            (ServerContext (..))
@@ -192,7 +193,7 @@ handleOffsetFetch ServerContext{..} reqCtx req = E.handle (\(K.ErrorCodeExceptio
   simpleAuthorize (toAuthorizableReqCtx reqCtx) authorizer Res_GROUP req.groupId AclOp_DESCRIBE >>= \case
     False -> return $ makeErrorResponse K.TOPIC_AUTHORIZATION_FAILED
     True  -> do
-      group <- GC.getGroup scGroupCoordinator req.groupId
+      group_m <- GC.getGroupM scGroupCoordinator req.groupId
       case K.unKaArray req.topics of
         -- 'Nothing' means fetch offsets of ALL topics.
         -- WARNING: Offsets of unauthzed topics should not be leaked
@@ -201,11 +202,13 @@ handleOffsetFetch ServerContext{..} reqCtx req = E.handle (\(K.ErrorCodeExceptio
         -- FIXME: Better method than passing a "validate" function?
         Nothing -> do
           Log.debug $ "fetching all offsets in group:" <> Log.build req.groupId
-          topicResps <- G.fetchAllOffsets group $ \reqTopic -> do
-            -- [ACL] check [DESCRIBE TOPIC] for each topic
-            simpleAuthorize (toAuthorizableReqCtx reqCtx) authorizer Res_TOPIC reqTopic.name AclOp_DESCRIBE >>= \case
-              False -> return K.TOPIC_AUTHORIZATION_FAILED
-              True  -> return K.NONE
+          topicResps <- case group_m of
+            Nothing    -> return (vectorToKaArray V.empty)
+            Just group -> G.fetchAllOffsets group $ \reqTopic -> do
+              -- [ACL] check [DESCRIBE TOPIC] for each topic
+              simpleAuthorize (toAuthorizableReqCtx reqCtx) authorizer Res_TOPIC reqTopic.name AclOp_DESCRIBE >>= \case
+                False -> return K.TOPIC_AUTHORIZATION_FAILED
+                True  -> return K.NONE
           return $ K.OffsetFetchResponse
             { throttleTimeMs = 0
             , errorCode = K.NONE
@@ -217,11 +220,15 @@ handleOffsetFetch ServerContext{..} reqCtx req = E.handle (\(K.ErrorCodeExceptio
             --       on each topic. That is why we pass a "validate"
             --       function to it.
             -- FIXME: Better method than passing a "validate" function?
-            G.fetchOffsets group reqTopic $ \reqTopic_ -> do
-              -- [ACL] check [DESCRIBE TOPIC] for each topic
-              simpleAuthorize (toAuthorizableReqCtx reqCtx) authorizer Res_TOPIC reqTopic_.name AclOp_DESCRIBE >>= \case
-                False -> return K.TOPIC_AUTHORIZATION_FAILED
-                True  -> return K.NONE
+            case group_m of
+              -- FIXME: what error code should it return? 'K.UNKNOWN_TOPIC_OR_PARTITION'
+              --        crashes some tests...
+              Nothing    -> return (makeErrorTopicResponse K.NONE reqTopic)
+              Just group -> G.fetchOffsets group reqTopic $ \reqTopic_ -> do
+                -- [ACL] check [DESCRIBE TOPIC] for each topic
+                simpleAuthorize (toAuthorizableReqCtx reqCtx) authorizer Res_TOPIC reqTopic_.name AclOp_DESCRIBE >>= \case
+                  False -> return K.TOPIC_AUTHORIZATION_FAILED
+                  True  -> return K.NONE
           return $ K.OffsetFetchResponse
             { throttleTimeMs = 0
             , errorCode = K.NONE
@@ -240,7 +247,7 @@ handleOffsetFetch ServerContext{..} reqCtx req = E.handle (\(K.ErrorCodeExceptio
             K.OffsetFetchResponsePartition
             { partitionIndex  = idx
             , errorCode       = code
-            , metadata        = Nothing
+            , metadata        = Just ""
             , committedOffset = -1
             }
       }