diff --git a/fastapi/routing.py b/fastapi/routing.py
index a4c25c159..e8e5ba788 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -500,7 +500,9 @@ class APIRouter(routing.Router):
             ), "A path prefix must not end with '/', as the routes will start with '/'"
         self.prefix = prefix
         if tags:
-            assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
+            assert not isinstance(
+                tags, str
+            ), """tags should not be a string: please use ["tag"] instead of "tag"."""
         self.tags: List[Union[str, Enum]] = tags or []
         self.dependencies = list(dependencies or []) or []
         self.deprecated = deprecated
@@ -554,7 +556,9 @@ class APIRouter(routing.Router):
         )
         current_tags = self.tags.copy()
         if tags:
-            assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
+            assert not isinstance(
+                tags, str
+            ), """tags should not be a string: please use ["tag"] instead of "tag"."""
             current_tags.extend(tags)
         current_dependencies = self.dependencies.copy()
         if dependencies:
@@ -719,7 +723,9 @@ class APIRouter(routing.Router):
                 )
                 current_tags = []
                 if tags:
-                    assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
+                    assert not isinstance(
+                        tags, str
+                    ), """tags should not be a string: please use ["tag"] instead of "tag"."""
                     current_tags.extend(tags)
                 if route.tags:
                     current_tags.extend(route.tags)
diff --git a/tests/test_tags.py b/tests/test_tags.py
index f38d84223..e115b3e3d 100644
--- a/tests/test_tags.py
+++ b/tests/test_tags.py
@@ -6,10 +6,12 @@ def test_string_is_invalid_in_router_tags():
     with pytest.raises(AssertionError):
         router = APIRouter(tags="test")
 
+
 def test_string_is_invalid_in_router_route_tags():
     router = APIRouter()
 
     with pytest.raises(AssertionError):
+
         @router.get("", tags="test")
         def test():
             ...
@@ -24,4 +26,4 @@ def test_string_is_invalid_in_include_router_tags():
         ...
 
     with pytest.raises(AssertionError):
-        app.include_router(router, prefix="/test", tags="test")
\ No newline at end of file
+        app.include_router(router, prefix="/test", tags="test")