diff --git a/borg/repository.py b/borg/repository.py
index 230c7d866..bf7a27a15 100644
--- a/borg/repository.py
+++ b/borg/repository.py
@@ -175,7 +175,7 @@ class Repository:
             # the repository instance lives on - even if exceptions happened.
             self._active_txn = False
             raise
-        if not self.index:
+        if not self.index or transaction_id is None:
             self.index = self.open_index(transaction_id)
         if transaction_id is None:
             self.segments = {}
@@ -237,38 +237,41 @@ class Repository:
 
     def replay_segments(self, index_transaction_id, segments_transaction_id):
         self.prepare_txn(index_transaction_id, do_cleanup=False)
-        for segment, filename in self.io.segment_iterator():
-            if index_transaction_id is not None and segment <= index_transaction_id:
-                continue
-            if segment > segments_transaction_id:
-                break
-            self.segments[segment] = 0
-            for tag, key, offset in self.io.iter_objects(segment):
-                if tag == TAG_PUT:
-                    try:
-                        s, _ = self.index[key]
-                        self.compact.add(s)
-                        self.segments[s] -= 1
-                    except KeyError:
-                        pass
-                    self.index[key] = segment, offset
-                    self.segments[segment] += 1
-                elif tag == TAG_DELETE:
-                    try:
-                        s, _ = self.index.pop(key)
-                        self.segments[s] -= 1
-                        self.compact.add(s)
-                    except KeyError:
-                        pass
-                    self.compact.add(segment)
-                elif tag == TAG_COMMIT:
+        try:
+            for segment, filename in self.io.segment_iterator():
+                if index_transaction_id is not None and segment <= index_transaction_id:
                     continue
-                else:
-                    raise self.CheckNeeded(self.path)
-            if self.segments[segment] == 0:
-                self.compact.add(segment)
-        self.write_index()
-        self.rollback()
+                if segment > segments_transaction_id:
+                    break
+                # code duplication below?? vvv (see similar code in check())
+                self.segments[segment] = 0
+                for tag, key, offset in self.io.iter_objects(segment):
+                    if tag == TAG_PUT:
+                        try:
+                            s, _ = self.index[key]
+                            self.compact.add(s)
+                            self.segments[s] -= 1
+                        except KeyError:
+                            pass
+                        self.index[key] = segment, offset
+                        self.segments[segment] += 1
+                    elif tag == TAG_DELETE:
+                        try:
+                            s, _ = self.index.pop(key)
+                            self.segments[s] -= 1
+                            self.compact.add(s)
+                        except KeyError:
+                            pass
+                        self.compact.add(segment)
+                    elif tag == TAG_COMMIT:
+                        continue
+                    else:
+                        raise self.CheckNeeded(self.path)
+                if self.segments[segment] == 0:
+                    self.compact.add(segment)
+            self.write_index()
+        finally:
+            self.rollback()
 
     def check(self, repair=False):
         """Check repository consistency
@@ -297,7 +300,7 @@ class Repository:
         if repair:
             self.io.cleanup(transaction_id)
         segments_transaction_id = self.io.get_segments_transaction_id()
-        self.prepare_txn(None)
+        self.prepare_txn(None)  # self.index, self.compact, self.segments all empty now!
         for segment, filename in self.io.segment_iterator():
             if segment > transaction_id:
                 continue