diff --git a/py/orbit/matrix_lattice/MATRIX_Lattice.py b/py/orbit/matrix_lattice/MATRIX_Lattice.py
index 22380391..afbadd2b 100644
--- a/py/orbit/matrix_lattice/MATRIX_Lattice.py
+++ b/py/orbit/matrix_lattice/MATRIX_Lattice.py
@@ -248,6 +248,8 @@ def trackTwissData(self, alpha, beta, direction="x"):
                 alpha_0 = track_v.get(0)
                 beta_0 = track_v.get(1)
                 delta_phi = math.atan(mt.get(ind0, ind1) / (beta_0 * mt.get(ind0, ind0) - alpha_0 * mt.get(ind0, ind1)))
+                if delta_phi < 0:
+                    delta_phi += math.pi
                 phi = phi + delta_phi
                 track_v = track_m.mult(track_v)
                 position = position + matrixNode.getLength()