diff --git a/testing/web-platform/tests/tools/webdriver/webdriver/client.py b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
index 470677f5fa730..9d3171f6af2da 100644
--- a/testing/web-platform/tests/tools/webdriver/webdriver/client.py
+++ b/testing/web-platform/tests/tools/webdriver/webdriver/client.py
@@ -1,10 +1,11 @@
+from typing import Dict
+from urllib import parse as urlparse
+
 from . import error
 from . import protocol
 from . import transport
 from .bidi.client import BidiSession
 
-from urllib import parse as urlparse
-
 
 def command(func):
     def inner(self, *args, **kwargs):
@@ -551,6 +552,9 @@ def start(self):
             body["capabilities"] = self.requested_capabilities
 
         value = self.send_command("POST", "session", body=body)
+        assert isinstance(value["sessionId"], str)
+        assert isinstance(value["capabilities"], Dict)
+
         self.session_id = value["sessionId"]
         self.capabilities = value["capabilities"]
 
diff --git a/testing/web-platform/tests/webdriver/tests/support/fixtures.py b/testing/web-platform/tests/webdriver/tests/support/fixtures.py
index e42b0ad9a26fd..bcffecc44c2b6 100644
--- a/testing/web-platform/tests/webdriver/tests/support/fixtures.py
+++ b/testing/web-platform/tests/webdriver/tests/support/fixtures.py
@@ -113,12 +113,8 @@ async def session(capabilities, configuration, request):
             configuration["host"],
             configuration["port"],
             capabilities=caps)
-    try:
-        _current_session.start()
 
-    except webdriver.error.SessionNotCreatedException:
-        if not _current_session.session_id:
-            raise
+    _current_session.start()
 
     # Enforce a fixed default window size and position
     if _current_session.capabilities.get("setWindowRect"):
@@ -159,12 +155,8 @@ async def bidi_session(capabilities, configuration, request):
             capabilities=caps,
             enable_bidi=True)
 
-    try:
-        _current_session.start()
-        await _current_session.bidi_session.start()
-    except webdriver.error.SessionNotCreatedException:
-        if not _current_session.session_id:
-            raise
+    _current_session.start()
+    await _current_session.bidi_session.start()
 
     # Enforce a fixed default window size and position
     _current_session.window.size = defaults.WINDOW_SIZE