diff --git a/postpic/particles/scalarproperties.py b/postpic/particles/scalarproperties.py
index 2055d672..3b23b7a8 100644
--- a/postpic/particles/scalarproperties.py
+++ b/postpic/particles/scalarproperties.py
@@ -172,8 +172,8 @@ def __call__(self, expr):
     ScalarProperty('z', 'z', 'm'),
     ScalarProperty('y * 1e6', 'z_um', r'$\mu$ m'),
     ScalarProperty('px', 'px', 'kg*m/s'),
-    ScalarProperty('py', 'pz', 'kg*m/s'),
-    ScalarProperty('pz', 'py', 'kg*m/s'),
+    ScalarProperty('py', 'py', 'kg*m/s'),
+    ScalarProperty('pz', 'pz', 'kg*m/s'),
     ScalarProperty('sqrt(px**2 + py**2 + pz**2)', 'p', 'kg*m/s'),
     ScalarProperty('(px**2 + py**2 + pz**2)/(mass * c)**2', '_np2', ''),
     ScalarProperty('_np2 / (sqrt(1 + _np2) + 1)', 'gamma_m1', ''),