diff --git a/evcouplings/compare/pdb.py b/evcouplings/compare/pdb.py
index c76ba25..e16aaf1 100644
--- a/evcouplings/compare/pdb.py
+++ b/evcouplings/compare/pdb.py
@@ -457,7 +457,7 @@ def _get_range(object_counts):
         # store explicit information about composition of residues
         def _group_info(field):
             return np.array(
-                [x[field] for x in mmtf.group_list]
+                [x[field] for x in mmtf.group_list], dtype=np.object_
             )
 
         # three and one letter code names of different groups
@@ -589,7 +589,7 @@ def get_chain(self, chain, model=0):
             np.array([
                 np.arange(self.first_residue_index[i], self.last_residue_index[i])
                 for i in target_chain_indeces
-            ])
+            ], dtype=np.object_)
         )
 
         # chain indeces and identifiers for all residues