diff --git a/km3io/tools.py b/km3io/tools.py
index 80b612e9d6d2ffd5bf484f84071fa3e8d3d2260c..619a877fdbdbfa874a819079b135a56a6894a059 100644
--- a/km3io/tools.py
+++ b/km3io/tools.py
@@ -472,7 +472,9 @@ def is_cc(fobj):
             cc_flag = w2list[:, kw2gen.W2LIST_GENHEN_CC]
             out = cc_flag > 0
         else:
-            raise NotImplementedError(f"don't know how to determine the CCness of {program} files.")
+            raise NotImplementedError(
+                f"don't know how to determine the CC-ness of {program} files."
+            )
 
     return out