diff --git a/python/corsika/io/outputs/interaction.py b/python/corsika/io/outputs/interaction.py
index 21a1b1d54650066d2d3e664238e8dc3f6286d154..956d1d75bfbdf5b8874fc73d018f56e561e427f6 100644
--- a/python/corsika/io/outputs/interaction.py
+++ b/python/corsika/io/outputs/interaction.py
@@ -36,9 +36,6 @@ class FirstInteraction(Particle):
         """
         Particle.__init__(self, prop_dict)
 
-        # need to define for momentum prop
-        self.px = self.py = self.pz = None
-
     def __repr__(self) -> str:
         """
         Return a string representation of this class.
diff --git a/python/corsika/io/outputs/primary.py b/python/corsika/io/outputs/primary.py
index d0105f1a0f9285a17e0d6d2acfe5afd56a044ce7..d5ee863cef4b8e7340b78605ce6c57872387f67b 100644
--- a/python/corsika/io/outputs/primary.py
+++ b/python/corsika/io/outputs/primary.py
@@ -34,6 +34,7 @@ class Particle(object):
         # need to define for position/direction functions
         self.x = self.y = self.z = None
         self.nx = self.ny = self.nz = None
+        self.px = self.py = self.pz = None
 
         # wrap the files of the dictionary so that there is a property
         # assigned to each value, i.e. print(my_primary.pdg)