diff --git a/fastapi/routing.py b/fastapi/routing.py
index fa1351859..8842745bd 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -1204,6 +1204,7 @@ class APIRouter(routing.Router):
         app.include_router(internal_router)
         ```
         """
+        assert self != router, "Router cannot be the same as parent"
         if prefix:
             assert prefix.startswith("/"), "A path prefix must start with '/'"
             assert not prefix.endswith(