diff --git a/swh/provenance/mongo/backend.py b/swh/provenance/mongo/backend.py
--- a/swh/provenance/mongo/backend.py
+++ b/swh/provenance/mongo/backend.py
@@ -1,4 +1,4 @@
-# Copyright (C) 2021  The Software Heritage developers
+# Copyright (C) 2021-2022  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
@@ -8,11 +8,20 @@
 from datetime import datetime, timezone
 import os
 from types import TracebackType
-from typing import Any, Dict, Generator, Iterable, List, Optional, Set, Type, Union
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Dict,
+    Generator,
+    Iterable,
+    List,
+    Optional,
+    Set,
+    Type,
+    Union,
+)
 
 from bson import ObjectId
-import mongomock
-import pymongo
 
 from swh.core.statsd import statsd
 from swh.model.model import Sha1Git
@@ -29,6 +38,9 @@
 
 STORAGE_DURATION_METRIC = "swh_provenance_storage_mongodb_duration_seconds"
 
+if TYPE_CHECKING:
+    from pymongo.database import Database
+
 
 class ProvenanceStorageMongoDb:
     def __init__(self, engine: str, **kwargs):
@@ -96,6 +108,8 @@
             else:
                 origin = {"url": None}
 
+            assert origin is not None
+
             for path in content["revision"][str(revision["_id"])]:
                 occurs.append(
                     ProvenanceResult(
@@ -125,6 +139,8 @@
             else:
                 origin = {"url": None}
 
+            assert origin is not None
+
             for path in content["revision"][str(revision["_id"])]:
                 occurs.append(
                     ProvenanceResult(
@@ -146,6 +162,8 @@
                 else:
                     origin = {"url": None}
 
+                assert origin is not None
+
                 for suffix in content["directory"][str(directory["_id"])]:
                     for prefix in directory["revision"][str(revision["_id"])]:
                         path = (
@@ -241,10 +259,10 @@
     @statsd.timed(metric=STORAGE_DURATION_METRIC, tags={"method": "open"})
     def open(self) -> None:
         if self.engine == "mongomock":
-            self.db = mongomock.MongoClient(**self.conn_args).get_database(self.dbname)
-        else:
-            # assume real MongoDB server by default
-            self.db = pymongo.MongoClient(**self.conn_args).get_database(self.dbname)
+            from mongomock import MongoClient as MongoClient
+        else:  # assume real MongoDB server by default
+            from pymongo import MongoClient
+        self.db: Database = MongoClient(**self.conn_args).get_database(self.dbname)
 
     @statsd.timed(metric=STORAGE_DURATION_METRIC, tags={"method": "origin_add"})
     def origin_add(self, orgs: Dict[Sha1Git, str]) -> bool: