--- a/testing/web-platform/tests/tools/wptserve/wptserve/server.py
+++ b/testing/web-platform/tests/tools/wptserve/wptserve/server.py
@@ -394,19 +394,20 @@ class WebTestHttpd(object):
 
         self.use_ssl = use_ssl
         self.logger = get_logger()
 
         if server_cls is None:
             server_cls = WebTestServer
 
         if use_ssl:
-            if key_file is not None:
-                assert os.path.exists(key_file)
-            assert certificate is not None and os.path.exists(certificate)
+            if not os.path.exists(key_file):
+                raise ValueError("SSL certificate not found: {}".format(key_file))
+            if not os.path.exists(certificate):
+                raise ValueError("SSL key not found: {}".format(certificate))
 
         try:
             self.httpd = server_cls((host, port),
                                     handler_cls,
                                     self.router,
                                     self.rewriter,
                                     config=config,
                                     bind_hostname=bind_hostname,