diff --git a/km3io/aanet.py b/km3io/aanet.py
index 5f771eabcfc8a682f19cd508e0f125d0ce262859..4e5317a40ec184161fd215f7027706c8b415a9a7 100644
--- a/km3io/aanet.py
+++ b/km3io/aanet.py
@@ -45,7 +45,8 @@ class AanetReader:
             branches are not read.
         """
         if key not in self.keys() and not isinstance(key, int):
-            raise KeyError("'{}' is not a valid key or is a fake branch.".format(key))
+            raise KeyError(
+                "'{}' is not a valid key or is a fake branch.".format(key))
         return self.lazy_data[key]
 
     def __len__(self):