diff --git a/swiftsimio/masks.py b/swiftsimio/masks.py
index 2bd560e6..028b7853 100644
--- a/swiftsimio/masks.py
+++ b/swiftsimio/masks.py
@@ -521,9 +521,9 @@ def constrain_index(self, index: int):
         if not self.metadata.filetype == "SOAP":
             warnings.warn("Not masking a SOAP catalogue, nothing constrained.")
             return
-        for group_name in self.metadata.present_group_names:
-            setattr(self, group_name, np.array([[index, index + 1]]))
-            setattr(self, f"{group_name}_size", 1)
+        self._shared = np.array([[index, index + 1]])
+        self._shared_size = 1
+
         return
 
     def get_masked_counts_offsets(