diff --git a/responder/routes.py b/responder/routes.py
index 954e858..13db9f6 100644
--- a/responder/routes.py
+++ b/responder/routes.py
@@ -457,7 +457,7 @@ class Router:
         # Call into a submounted app, if one exists.
         for path_prefix, app in self.apps.items():
             if path.startswith(path_prefix):
-                scope["path"] = path[len(path_prefix) :]
+                scope["path"] = path[len(path_prefix) :] or "/"
                 scope["root_path"] = root_path + path_prefix
                 try:
                     await app(scope, receive, send)
diff --git a/tests/test_responder.py b/tests/test_responder.py
index e4ef3f8..b2f9375 100644
--- a/tests/test_responder.py
+++ b/tests/test_responder.py
@@ -546,14 +546,17 @@ def test_documentation(needs_openapi):
     assert "html" in r.text
 
 
-def test_mount_wsgi_app(api, flask):
+def test_mount_wsgi_app(flask):
+    # Use localhost so Werkzeug's trusted-host check accepts the request.
+    api = responder.API(allowed_hosts=["localhost"])
+
     @api.route("/")
     def hello(req, resp):
         resp.text = "hello"
 
     api.mount("/flask", flask)
 
-    r = api.requests.get("http://;/flask")
+    r = api.requests.get("http://localhost/flask")
     assert r.status_code < 300