diff --git a/swh/loader/git/converters.py b/swh/loader/git/converters.py
--- a/swh/loader/git/converters.py
+++ b/swh/loader/git/converters.py
@@ -94,11 +94,11 @@
     Software Heritage"""
     return TimestampWithTimezone(
         timestamp=Timestamp(
-            seconds=timestamp,
+            seconds=int(timestamp),
             microseconds=0,
         ),
         offset=timezone // 60,
-        negative_utc=timezone_neg_utc if timezone == 0 else None,
+        negative_utc=timezone_neg_utc if timezone == 0 else False,
     )
 
 
diff --git a/swh/loader/git/tests/test_converters.py b/swh/loader/git/tests/test_converters.py
--- a/swh/loader/git/tests/test_converters.py
+++ b/swh/loader/git/tests/test_converters.py
@@ -143,7 +143,7 @@
                     seconds=1443083765,
                     microseconds=0,
                 ),
-                negative_utc=None,
+                negative_utc=False,
                 offset=120,
             ),
             message=b'add submodule dependency\n',
@@ -153,7 +153,7 @@
                     seconds=1443083765,
                     microseconds=0,
                 ),
-                negative_utc=None,
+                negative_utc=False,
                 offset=120,
             ),
             parents=[
@@ -215,7 +215,7 @@
     def test_dulwich_tag_to_release_no_author_no_date(self):
         target = b'641fb6e08ddb2e4fd096dcf18e80b894bf'
         message = b'some release message'
-        tag = SWHTag(name='blah',
+        tag = SWHTag(name=b'blah',
                      type_name=b'tag',
                      target=target,
                      target_type=b'commit',
@@ -233,7 +233,7 @@
             id=b'\xda9\xa3\xee^kK\r2U\xbf\xef\x95`\x18\x90\xaf\xd8\x07\t',
             message=message,
             metadata=None,
-            name='blah',
+            name=b'blah',
             synthetic=False,
             target=hash_to_bytes(target.decode()),
             target_type=ObjectType.REVISION,
@@ -251,7 +251,7 @@
             2007, 12, 5, tzinfo=datetime.timezone.utc
         ).timestamp()
 
-        tag = SWHTag(name='blah',
+        tag = SWHTag(name=b'blah',
                      type_name=b'tag',
                      target=target,
                      target_type=b'commit',
@@ -281,7 +281,7 @@
             id=b'\xda9\xa3\xee^kK\r2U\xbf\xef\x95`\x18\x90\xaf\xd8\x07\t',
             message=message,
             metadata=None,
-            name='blah',
+            name=b'blah',
             synthetic=False,
             target=hash_to_bytes(target.decode()),
             target_type=ObjectType.REVISION,
@@ -294,7 +294,7 @@
         tagger = b'hey dude <hello@mail.org>'
         target = b'641fb6e08ddb2e4fd096dcf18e80b894bf'
         message = b'some release message'
-        tag = SWHTag(name='blah',
+        tag = SWHTag(name=b'blah',
                      type_name=b'tag',
                      target=target,
                      target_type=b'commit',
@@ -316,7 +316,7 @@
             id=b'\xda9\xa3\xee^kK\r2U\xbf\xef\x95`\x18\x90\xaf\xd8\x07\t',
             message=message,
             metadata=None,
-            name='blah',
+            name=b'blah',
             synthetic=False,
             target=hash_to_bytes(target.decode()),
             target_type=ObjectType.REVISION,