diff --git a/ess/TraceWin.py b/ess/TraceWin.py
index a0538a5409d23d33b01bce86a9b8453a3eae8b4f..a585e622d0e9d1c65f20b7ee70c811c12cfa3bf0 100644
--- a/ess/TraceWin.py
+++ b/ess/TraceWin.py
@@ -508,11 +508,11 @@ class plt:
 
         # Calculate beta:
         # This is a factor 10 different from what TraceWin plots
-        self.twiss_beta = [[self.sigma[j][i][i] / self.twiss_eps[j, i / 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
+        self.twiss_beta = [[self.sigma[j][i][i] / self.twiss_eps[j, i // 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
         self.twiss_beta = numpy.array(self.twiss_beta)
 
         # Calculate alpha:
-        self.twiss_alpha = [[-self.sigma[j][i][i + 1] / self.twiss_eps[j, i / 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
+        self.twiss_alpha = [[-self.sigma[j][i][i + 1] / self.twiss_eps[j, i // 2] for i in (0, 2, 4)] for j in range(len(self.Nelp))]
         self.twiss_alpha = numpy.array(self.twiss_alpha)
 
     def get_dst(self, index):