diff --git a/swiftsimio/metadata/objects.py b/swiftsimio/metadata/objects.py
index b47a7938..db0cdcec 100644
--- a/swiftsimio/metadata/objects.py
+++ b/swiftsimio/metadata/objects.py
@@ -377,7 +377,9 @@ def __init__(self, base_mass_table: np.array, mass_units: unyt.unyt_quantity):
 
         # TODO: Extract these names from the files themselves if possible.
 
-        for index, name in metadata.particle_types.particle_name_underscores.items():
+        for index, name in enumerate(
+            metadata.particle_types.particle_name_underscores.values()
+        ):
             try:
                 setattr(
                     self,