diff --git a/python/vasp/plotools.py b/python/vasp/plotools.py
index 27ffa860..d86caa67 100644
--- a/python/vasp/plotools.py
+++ b/python/vasp/plotools.py
@@ -81,6 +81,7 @@ def check_data_consistency(pars, el_struct):
     """
 # Check that ions inside each shell are of the same sort
     for sh in pars.shells:
+        assert max(sh['ion_list']) <= el_struct.natom, "Site index in the projected shell exceeds the number of ions in the structure"
         sorts = set([el_struct.type_of_ion[io] for io in sh['ion_list']])
         assert len(sorts) == 1, "Each projected shell must contain only ions of the same sort"