diff --git a/deepszsim/simclusters.py b/deepszsim/simclusters.py
index 75ee5c0..3db4f7a 100644
--- a/deepszsim/simclusters.py
+++ b/deepszsim/simclusters.py
@@ -101,7 +101,7 @@ def __init__(self, M200 = None, redshift_z = None, num_halos = None, halo_params
                  for i in range(self._size)]).T
         
         self.id_list = [
-            str(int(self.M200[i]//1e9)).zfill(6) + "_" + str(int(self.redshift_z[i]*100)).zfill(3) + "_" + str(
+            str(int(self.M200[i]//1e9)).zfill(7) + "_" + str(int(self.redshift_z[i]*100)).zfill(3) + "_" + str(
                 self._rng.integers(
                 10**6)).zfill(6)
             for i in range(self._size)]