diff --git a/fastapi/applications.py b/fastapi/applications.py
index 4af1146b0d..d5829ad03c 100644
--- a/fastapi/applications.py
+++ b/fastapi/applications.py
@@ -1104,7 +1104,14 @@ class FastAPI(Starlette):
             async def openapi(req: Request) -> JSONResponse:
                 root_path = req.scope.get("root_path", "").rstrip("/")
                 schema = self.openapi()
-                if root_path and self.root_path_in_servers:
+                if self.root_path_in_servers and "servers" not in schema:
+                    if root_path:
+                        schema = dict(schema)
+                        schema["servers"] = [{"url": root_path}]
+                    else:
+                        schema = dict(schema)
+                        schema["servers"] = [{"url": "/"}]
+                elif root_path and self.root_path_in_servers:
                     server_urls = {s.get("url") for s in schema.get("servers", [])}
                     if root_path not in server_urls:
                         schema = dict(schema)