diff --git a/tracking/element.py b/tracking/element.py
index 9123e901224ce2d8e399ffa9ba9fbae603f2e9fe..519190eed15bc2418aed71e4474255e07ad85d47 100644
--- a/tracking/element.py
+++ b/tracking/element.py
@@ -93,7 +93,7 @@ class LongitudinalMap(Element):
         bunch : Bunch or Beam object
         """
         bunch["delta"] -= self.ring.U0 / self.ring.E0
-        bunch["tau"] -= self.ring.ac * self.ring.T0 * bunch["delta"]
+        bunch["tau"] += self.ring.ac * self.ring.T0 * bunch["delta"]
 
 class SynchrotronRadiation(Element):
     """