diff --git a/swh/scanner/scanner.py b/swh/scanner/scanner.py
--- a/swh/scanner/scanner.py
+++ b/swh/scanner/scanner.py
@@ -100,24 +100,26 @@
 
     """
 
-    def swhid_of(path):
+    def swhid_of(path: Path) -> str:
         if path.is_dir():
             if exclude_patterns:
 
-                def dir_filter(dirpath, *args):
+                def dir_filter(dirpath: str, *args) -> bool:
                     return directory_filter(dirpath, exclude_patterns)
 
             else:
-                dir_filter = accept_all_directories
+                dir_filter = accept_all_directories  # type: ignore
 
             obj = Directory.from_disk(
                 path=bytes(path), dir_filter=dir_filter
             ).get_data()
 
-            return CoreSWHID(object_type=ObjectType.DIRECTORY, object_id=obj["id"])
+            return str(CoreSWHID(object_type=ObjectType.DIRECTORY, object_id=obj["id"]))
         else:
             obj = Content.from_file(path=bytes(path)).get_data()
-            return Content(object_type=ObjectType.CONTENT, object_id=obj["sha1_git"])
+            return str(
+                CoreSWHID(object_type=ObjectType.CONTENT, object_id=obj["sha1_git"])
+            )
 
     dirpath, dnames, fnames = next(os.walk(path))
     for node in itertools.chain(dnames, fnames):
diff --git a/swh/scanner/tests/flask_api.py b/swh/scanner/tests/flask_api.py
--- a/swh/scanner/tests/flask_api.py
+++ b/swh/scanner/tests/flask_api.py
@@ -13,6 +13,10 @@
 def create_app():
     app = Flask(__name__)
 
+    @app.route("/")
+    def index():
+        return "SWH scanner API"
+
     @app.route("/known/", methods=["POST"])
     def known():
         swhids = request.get_json()
diff --git a/swh/scanner/tests/test_scanner.py b/swh/scanner/tests/test_scanner.py
--- a/swh/scanner/tests/test_scanner.py
+++ b/swh/scanner/tests/test_scanner.py
@@ -5,6 +5,7 @@
 
 import json
 
+from flask import url_for
 import pytest
 
 from swh.scanner.exceptions import APIError, InvalidDirectoryPath
@@ -54,7 +55,7 @@
 
 def test_scanner_raise_apierror_input_size_limit(event_loop, aiosession, live_server):
 
-    api_url = live_server.url() + "/"
+    api_url = url_for("index", _external=True)
     request = [
         "swh:1:cnt:7c4c57ba9ff496ad179b8f65b1d286edbda34c9a" for i in range(901)
     ]  # /known/ is limited at 900
@@ -82,7 +83,7 @@
 
 
 def test_scanner_result(live_server, event_loop, test_sample_folder):
-    api_url = live_server.url() + "/"
+    api_url = url_for("index", _external=True)
     config = {"web-api": {"url": api_url, "auth-token": None}}
 
     source_tree = Tree(test_sample_folder)
@@ -99,7 +100,7 @@
 def test_scanner_result_with_exclude_patterns(
     live_server, event_loop, test_sample_folder
 ):
-    api_url = live_server.url() + "/"
+    api_url = url_for("index", _external=True)
     config = {"web-api": {"url": api_url, "auth-token": None}}
 
     patterns = (str(test_sample_folder) + "/toexclude",)