diff --git a/swh/storage/db.py b/swh/storage/db.py
--- a/swh/storage/db.py
+++ b/swh/storage/db.py
@@ -452,11 +452,6 @@
             'SELECT %s' % ', '.join(self.origin_visit_get_cols),
             'FROM origin_visit']
 
-        if require_snapshot:
-            # Makes sure the snapshot is known
-            query_parts.append(
-                'INNER JOIN snapshot ON (origin_visit.snapshot=snapshot.id)')
-
         query_parts.append('WHERE origin = %s')
 
         if require_snapshot:
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -816,7 +816,11 @@
         visit = self.origin_visit_get_latest(
             origin, allowed_statuses=allowed_statuses, require_snapshot=True)
         if visit and visit['snapshot']:
-            return self.snapshot_get(visit['snapshot'])
+            snapshot = self.snapshot_get(visit['snapshot'])
+            if not snapshot:
+                raise ValueError(
+                    'last origin visit references an unknown snapshot')
+            return snapshot
 
     def snapshot_count_branches(self, snapshot_id, db=None, cur=None):
         """Count the number of branches in the snapshot with the given id
@@ -1363,8 +1367,7 @@
                       if visit['status'] in allowed_statuses]
         if require_snapshot:
             visits = [visit for visit in visits
-                      if visit['snapshot']
-                      and visit['snapshot'] in self._snapshots]
+                      if visit['snapshot']]
 
         return max(visits, key=lambda v: (v['date'], v['visit']), default=None)
 
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -1070,7 +1070,12 @@
             origin, allowed_statuses=allowed_statuses, require_snapshot=True,
             db=db, cur=cur)
         if origin_visit and origin_visit['snapshot']:
-            return self.snapshot_get(origin_visit['snapshot'], db=db, cur=cur)
+            snapshot = self.snapshot_get(
+                    origin_visit['snapshot'], db=db, cur=cur)
+            if not snapshot:
+                raise ValueError(
+                    'last origin visit references an unknown snapshot')
+            return snapshot
 
     @db_transaction(statement_timeout=2000)
     def snapshot_count_branches(self, snapshot_id, db=None, cur=None):
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -2900,10 +2900,12 @@
         # Two visits, both with no snapshot: latest snapshot is None
         self.assertIsNone(self.storage.snapshot_get_latest(origin_id))
 
-        # Add unknown snapshot to visit1, latest snapshot = None
+        # Add unknown snapshot to visit1, check that the inconsistency is
+        # detected
         self.storage.origin_visit_update(
             origin_id, visit1_id, snapshot=self.complete_snapshot['id'])
-        self.assertIsNone(self.storage.snapshot_get_latest(origin_id))
+        with self.assertRaises(ValueError):
+            self.storage.snapshot_get_latest(origin_id)
 
         # Status filter: both visits are status=ongoing, so no snapshot
         # returned
@@ -2914,10 +2916,9 @@
 
         # Mark the first visit as completed and check status filter again
         self.storage.origin_visit_update(origin_id, visit1_id, status='full')
-        self.assertIsNone(
+        with self.assertRaises(ValueError):
             self.storage.snapshot_get_latest(origin_id,
                                              allowed_statuses=['full']),
-        )
 
         # Actually add the snapshot and check status filter again
         self.storage.snapshot_add([self.complete_snapshot])
@@ -2926,18 +2927,17 @@
             self.storage.snapshot_get_latest(origin_id)
         )
 
-        # Add unknown snapshot to visit2 and check that the old snapshot
-        # is still returned
+        # Add unknown snapshot to visit2 and check that the inconsistency
+        # is detected
         self.storage.origin_visit_update(
-            origin_id, visit2_id, snapshot=self.empty_snapshot['id'])
-        self.assertEqual(
-            self.complete_snapshot,
-            self.storage.snapshot_get_latest(origin_id))
+            origin_id, visit2_id, snapshot=self.snapshot['id'])
+        with self.assertRaises(ValueError):
+            self.storage.snapshot_get_latest(origin_id)
 
         # Actually add that snapshot and check that the new one is returned
-        self.storage.snapshot_add([self.empty_snapshot])
+        self.storage.snapshot_add([self.snapshot])
         self.assertEqual(
-            self.empty_snapshot,
+            self.snapshot,
             self.storage.snapshot_get_latest(origin_id)
         )