diff --git a/swh/journal/replay.py b/swh/journal/replay.py
--- a/swh/journal/replay.py
+++ b/swh/journal/replay.py
@@ -44,6 +44,17 @@
     return rev
 
 
+def _fix_revision_transplant_source(rev):
+    if rev.get('extra_headers'):
+        rev = rev.copy()
+        rev['extra_headers'] = [
+            [key, value.encode('ascii')]
+            if key == 'transplant_source' and isinstance(value, str)
+            else [key, value]
+            for (key, value) in rev['extra_headers']]
+    return rev
+
+
 def _check_date(date):
     """Returns whether the date can be represented in backends with sane
     limits on timestamps and timezeones (resp. signed 64-bits and
@@ -65,6 +76,7 @@
     good_revisions = []
     for rev in revisions:
         rev = _fix_revision_pypi_empty_string(rev)
+        rev = _fix_revision_transplant_source(rev)
         if not _check_revision_date(rev):
             logging.warning('Excluding revision (invalid date): %r', rev)
             continue
@@ -91,7 +103,7 @@
 
 
 def fix_objects(object_type, objects):
-    """Converts a possibly old object from the journal to its current
+    r"""Converts a possibly old object from the journal to its current
     expected format.
 
     List of conversions:
@@ -119,6 +131,21 @@
       'date': {'offset': 0,
                'timestamp': {'microseconds': 0, 'seconds': 1565096932}}}]
 
+    Fix type of 'transplant_source' extra headers:
+
+    >>> revs = fix_objects('revision', [{
+    ...     'author': {'email': '', 'fullname': b'', 'name': ''},
+    ...     'committer': {'email': '', 'fullname': b'', 'name': ''},
+    ...     'date': date,
+    ...     'committer_date': date,
+    ...     'extra_headers': [
+    ...         ['time_offset_seconds', b'-3600'],
+    ...         ['transplant_source', '29c154a012a70f49df983625090434587622b39e']]
+    ... }])
+    >>> pprint(revs[0]['extra_headers'])
+    [['time_offset_seconds', b'-3600'],
+     ['transplant_source', b'29c154a012a70f49df983625090434587622b39e']]
+
     Filter out revisions with invalid dates:
 
     >>> from copy import deepcopy