diff --git a/swh/objstorage/objstorage.py b/swh/objstorage/objstorage.py
--- a/swh/objstorage/objstorage.py
+++ b/swh/objstorage/objstorage.py
@@ -59,7 +59,7 @@
     'lzma': lzma.LZMADecompressor,
     'gzip': lambda: zlib.decompressobj(wbits=31),
     'zlib': zlib.decompressobj,
-    None: NullDecompressor,
+    'none': NullDecompressor,
 }
 
 compressors = {
@@ -67,8 +67,8 @@
     'lzma': lzma.LZMACompressor,
     'gzip': lambda: zlib.compressobj(wbits=31),
     'zlib': zlib.compressobj,
-    None: NullCompressor,
-    }
+    'none': NullCompressor,
+}
 
 
 class ObjStorage(metaclass=abc.ABCMeta):
diff --git a/swh/objstorage/tests/test_objstorage_azure.py b/swh/objstorage/tests/test_objstorage_azure.py
--- a/swh/objstorage/tests/test_objstorage_azure.py
+++ b/swh/objstorage/tests/test_objstorage_azure.py
@@ -7,7 +7,7 @@
 from collections import defaultdict
 from unittest.mock import patch
 
-from typing import Any, Dict, Optional
+from typing import Any, Dict
 
 from azure.common import AzureMissingResourceHttpError
 from swh.model.hashutil import hash_to_hex
@@ -69,7 +69,7 @@
 
 
 class TestAzureCloudObjStorage(ObjStorageTestFixture, unittest.TestCase):
-    compression = None  # type: Optional[str]
+    compression = 'none'
 
     def setUp(self):
         super().setUp()
@@ -109,13 +109,13 @@
 
         blob_service._data[container][internal_id] += b'trailing garbage'
 
-        if self.compression is not None:
+        if self.compression == 'none':
             with self.assertRaises(Error) as e:
-                self.storage.get(obj_id)
-            assert 'trailing data' in e.exception.args[0]
+                self.storage.check(obj_id)
         else:
             with self.assertRaises(Error) as e:
-                self.storage.check(obj_id)
+                self.storage.get(obj_id)
+            assert 'trailing data' in e.exception.args[0]
 
 
 class TestAzureCloudObjStorageGzip(TestAzureCloudObjStorage):
diff --git a/swh/objstorage/tests/test_objstorage_cloud.py b/swh/objstorage/tests/test_objstorage_cloud.py
--- a/swh/objstorage/tests/test_objstorage_cloud.py
+++ b/swh/objstorage/tests/test_objstorage_cloud.py
@@ -8,7 +8,6 @@
 from libcloud.common.types import InvalidCredsError
 from libcloud.storage.types import (ContainerDoesNotExistError,
                                     ObjectDoesNotExistError)
-from typing import Optional
 
 from swh.model import hashutil
 
@@ -95,7 +94,7 @@
 
 
 class TestCloudObjStorage(ObjStorageTestFixture, unittest.TestCase):
-    compression = None  # type: Optional[str]
+    compression = 'none'
 
     def setUp(self):
         super().setUp()
@@ -125,13 +124,13 @@
 
         data[obj_id].content.append(b'trailing garbage')
 
-        if self.compression is not None:
+        if self.compression == 'none':
             with self.assertRaises(Error) as e:
-                self.storage.get(obj_id)
-            assert 'trailing data' in e.exception.args[0]
+                self.storage.check(obj_id)
         else:
             with self.assertRaises(Error) as e:
-                self.storage.check(obj_id)
+                self.storage.get(obj_id)
+            assert 'trailing data' in e.exception.args[0]
 
 
 class TestCloudObjStorageBz2(TestCloudObjStorage):
diff --git a/swh/objstorage/tests/test_objstorage_pathslicing.py b/swh/objstorage/tests/test_objstorage_pathslicing.py
--- a/swh/objstorage/tests/test_objstorage_pathslicing.py
+++ b/swh/objstorage/tests/test_objstorage_pathslicing.py
@@ -8,8 +8,6 @@
 import unittest
 from unittest.mock import patch, DEFAULT
 
-from typing import Optional
-
 from swh.model import hashutil
 from swh.objstorage import exc, get_objstorage, ID_HASH_LENGTH
 
@@ -17,7 +15,7 @@
 
 
 class TestPathSlicingObjStorage(ObjStorageTestFixture, unittest.TestCase):
-    compression = None  # type: Optional[str]
+    compression = 'none'
 
     def setUp(self):
         super().setUp()
@@ -137,7 +135,7 @@
             f.write(b'garbage')
         with self.assertRaises(exc.Error) as error:
             self.storage.check(obj_id)
-        if self.compression is None:
+        if self.compression == 'none':
             self.assertIn('Corrupt object', error.exception.args[0])
         else:
             self.assertIn('trailing data found', error.exception.args[0])
diff --git a/swh/objstorage/tests/test_objstorage_seaweedfs.py b/swh/objstorage/tests/test_objstorage_seaweedfs.py
--- a/swh/objstorage/tests/test_objstorage_seaweedfs.py
+++ b/swh/objstorage/tests/test_objstorage_seaweedfs.py
@@ -5,8 +5,6 @@
 
 import unittest
 
-from typing import Optional
-
 from swh.objstorage.objstorage import decompressors
 from swh.objstorage.exc import Error
 
@@ -42,7 +40,7 @@
 
 
 class TestWeedObjStorage(ObjStorageTestFixture, unittest.TestCase):
-    compression = None  # type: Optional[str]
+    compression = 'none'
 
     def setUp(self):
         super().setUp()
@@ -68,13 +66,13 @@
         path = self.storage._path(obj_id)
         self.storage.wf.content[path] += b'trailing garbage'
 
-        if self.compression is not None:
+        if self.compression == 'none':
             with self.assertRaises(Error) as e:
-                self.storage.get(obj_id)
-            assert 'trailing data' in e.exception.args[0]
+                self.storage.check(obj_id)
         else:
             with self.assertRaises(Error) as e:
-                self.storage.check(obj_id)
+                self.storage.get(obj_id)
+            assert 'trailing data' in e.exception.args[0]
 
 
 class TestWeedObjStorageBz2(TestWeedObjStorage):