diff --git a/pytchat/tool/download/asyncdl.py b/pytchat/tool/download/asyncdl.py
index 57ea8c3..9d17692 100644
--- a/pytchat/tool/download/asyncdl.py
+++ b/pytchat/tool/download/asyncdl.py
@@ -1,4 +1,3 @@
-
 import aiohttp
 import asyncio
 import json
@@ -105,7 +104,7 @@ def download_patch(callback, blocks, video_id):
             if callback:
                 callback(actions, last - first)
             return Patch(actions, continuation, first, last)
-        return Patch()
+        return Patch(continuation = continuation)
     """
     allocate workers and assign blocks.
     """   
diff --git a/pytchat/tool/download/patch.py b/pytchat/tool/download/patch.py
index 211ff37..2305118 100644
--- a/pytchat/tool/download/patch.py
+++ b/pytchat/tool/download/patch.py
@@ -22,7 +22,6 @@ def fill(block:Block, patch:Patch):
         if line_offset < block_end:
             break
         patch.chats.pop()
-        
     set_patch(block, patch._replace(
         continuation = None,
         last = line_offset
@@ -34,15 +33,16 @@ def fill(block:Block, patch:Patch):
 
 def split(parent_block:Block, child_block:Block, patch:Patch):
     parent_block.during_split = False
-    """patch overlaps with parent_block"""
     if patch.first <= parent_block.last:
+        ''' When patch overlaps with parent_block,
+        discard this block. '''
         child_block.continuation = None
         ''' Leave child_block.during_split == True 
-         to exclude from during_split sequence.'''
+         to exclude from during_split sequence. '''
         return    
     child_block.during_split = False
-    child_block.first=patch.first
-    parent_block.end =patch.first
+    child_block.first = patch.first
+    parent_block.end = patch.first
     fill(child_block, patch)
     
 
diff --git a/tests/test_dl_duplcheck.py b/tests/test_dl_duplcheck.py
index 82d6e91..67fc00c 100644
--- a/tests/test_dl_duplcheck.py
+++ b/tests/test_dl_duplcheck.py
@@ -66,7 +66,7 @@ def test_duplicate_head():
                     [4] , [5]  -> append  [4]
                     append [5]
 
-        result    : [0] , [3] , [5] 
+        result    : [2] , [4] , [5] 
     """
 
     #chat data offsets are ignored.
diff --git a/tests/test_patch.py b/tests/test_patch.py
index 79f1353..4f5c208 100644
--- a/tests/test_patch.py
+++ b/tests/test_patch.py
@@ -23,6 +23,8 @@ def test_split_0():
     """
     Normal case
 
+    ~~~~~~ before ~~~~~~
+
      @parent_block  (# = already downloaded)
     
      first    last                                     end
@@ -31,18 +33,21 @@ def test_split_0():
 
      @child_block
     
-     first = last = 0                                  end=parent_end
-     ---------------------------------------------------|
+     first = last = 0                                  end (=parent_end)
+       |                                                |
     
 
      @fetched patch
                             |-- patch --|
     
      
-                             |
-                             |
-                             V 
+                            |
+                            |
+                            V 
+
+    ~~~~~~ after ~~~~~~
     
+
      @parent_block
     
      first    last         end (after split)   
@@ -83,20 +88,20 @@ def test_split_1():
     ~~~~~~ before ~~~~~~
 
                           patch.first
-      first                  |   last                  end
+      first                 |    last                  end
        |####################|#####|---------------------|
                             ^
      @child_block
-     first = last = 0                                  end=parent_end
-     ---------------------------------------------------|
+     first = last = 0                                  end (=parent_end)
+       |                                                |
      
      @fetched patch
                             |-- patch --|
     
      
-                             |
-                             |
-                             V 
+                            |
+                            |
+                            V 
     
     ~~~~~~ after ~~~~~~
 
@@ -135,8 +140,8 @@ def test_split_2():
        |########------------------------------|
     
      @child_block
-     first = last = 0                        end=parent_end
-     -----------------------------------------|
+     first = last = 0                        end (=parent_end)
+       |                                      |
     
     continuation:succeed from patch
     
@@ -144,9 +149,9 @@ def test_split_2():
                             |-------- patch --------|
     
      
-                             |
-                             |
-                             V 
+                            |
+                            |
+                            V 
 
     ~~~~~~ after ~~~~~~
 
@@ -196,23 +201,23 @@ def test_split_none():
        |####################|###################|-------|
                             ^
      @child_block
-     first = last = 0                                  end=parent_end
-     ---------------------------------------------------|
+     first = last = 0                                  end (=parent_end)
+       |                                                |
      
      @fetched patch
                             |-- patch --|
                                       patch.last < parent_block.last                       .
      
-                             |
-                             |
-                             V 
+                            |
+                            |
+                            V 
     
     ~~~~~~ after ~~~~~~
 
      @parent_block
      first                       last           end (before split)
        |########################################|-------|
-                                                              .
+
      @child_block
                                            
                             ............    -> discard all data.