diff --git a/org/qmckl_jastrow_champ.org b/org/qmckl_jastrow_champ.org
index 1f1dbad..8be1e7b 100644
--- a/org/qmckl_jastrow_champ.org
+++ b/org/qmckl_jastrow_champ.org
@@ -143,7 +143,7 @@ int main() {
     | ~bord_num~                | ~int64_t~                             | The number of b coeffecients                                      |
     | ~cord_num~                | ~int64_t~                             | The number of c coeffecients                                      |
     | ~type_nucl_num~           | ~int64_t~                             | Number of Nuclei types                                            |
-    | ~type_nucl_vector~        | ~int64_t[nucl_num]~                   | IDs of types of Nuclei                                            |
+    | ~type_nucl_vector~        | ~int64_t[nucl_num]~                   | IDs of types of Nuclei. These use 0-based indexing as in C.       |
     | ~a_vector~                | ~double[aord_num + 1][type_nucl_num]~ | a polynomial coefficients                                         |
     | ~b_vector~                | ~double[bord_num + 1]~                | b polynomial coefficients                                         |
     | ~c_vector~                | ~double[dim_c_vector][type_nucl_num]~ | c polynomial coefficients                                         |
@@ -578,6 +578,19 @@ qmckl_set_jastrow_champ_type_nucl_vector(qmckl_context context,
                            "type_nucl_vector = NULL");
   }
 
+  for (int i=0 ; i<nucl_num ; ++i) {
+    if (type_nucl_vector[i] < 0) {
+      return qmckl_failwith( context, QMCKL_INVALID_ARG_2,
+                             "qmckl_set_type_nucl_vector",
+                             "Inconsistent values of type_nucl_vector (<0)" );
+    }
+    if (type_nucl_vector[i] >= type_nucl_num) {
+      return qmckl_failwith( context, QMCKL_INVALID_ARG_2,
+                             "qmckl_set_type_nucl_vector",
+                             "Inconsistent values of type_nucl_vector (>=nucl_num). Values should use 0-based indexing as in C." );
+    }
+  }
+
   if (ctx->jastrow_champ.type_nucl_vector != NULL) {
     qmckl_exit_code rc = qmckl_free(context, ctx->jastrow_champ.type_nucl_vector);
     if (rc != QMCKL_SUCCESS) {