diff --git a/km3buu/output.py b/km3buu/output.py
index 1310d15b5dfd311576f9f2ae3f628badb8d1ab0e..4ae54f75d782fb45d99e6b763ba1bd83362b12ad 100644
--- a/km3buu/output.py
+++ b/km3buu/output.py
@@ -258,7 +258,7 @@ class GiBUUOutput:
 
     @property
     def mean_xsec(self):
-        if self.flux_data is None
+        if self.flux_data is None:
             return lambda energy: self.xsection["sum"]
         else
             root_tupledata = self.arrays