diff --git a/src/whitenoise/base.py b/src/whitenoise/base.py
index fea128f6..7ab9dcd8 100644
--- a/src/whitenoise/base.py
+++ b/src/whitenoise/base.py
@@ -153,18 +153,21 @@ def find_file(self, url):
         if settings.DEBUG and url.startswith(settings.STATIC_URL):
             paths = "\n    ".join(self.candidate_paths_for_url(url))
             from django.contrib.staticfiles.finders import get_finders
+
             finders = get_finders()
             app_dirs = []
             for finder in finders:
                 for storage in finder.storages.values():
                     app_dirs.append(storage.location)
             app_dirs = "\n    ".join(sorted(app_dirs))
-            raise MissingFileError(f"""{url} not found. Searched these paths:
+            raise MissingFileError(
+                f"""{url} not found. Searched these paths:
 
     {paths}
 
 App dirs:
-    {app_dirs}""")
+    {app_dirs}"""
+            )
 
     def candidate_paths_for_url(self, url):
         for root, prefix in self.directories: