diff --git a/mbtrack2/tracking/particles.py b/mbtrack2/tracking/particles.py
index 7c42a5bc9643dbc34346f5272796e5509a06303a..8a50004481ffe1003c7ca7014cd95b69f6a9c4a4 100644
--- a/mbtrack2/tracking/particles.py
+++ b/mbtrack2/tracking/particles.py
@@ -317,7 +317,7 @@ class Bunch:
 
         """
         xb = self['x'] - self['delta'] * self.ring.optics.local_dispersion[0]
-        yy = self['y'] - self['delta'] * self.ring.optics.local_dispersion[2]
+        yb = self['y'] - self['delta'] * self.ring.optics.local_dispersion[2]
         xpb = self['xp'] - self['delta'] * self.ring.optics.local_dispersion[1]
         ypb = self['yp'] - self['delta'] * self.ring.optics.local_dispersion[3]
         Jx = (self.ring.optics.local_gamma[0] * xb**2) + \