diff --git a/thunk.c b/thunk.c
index 2dac36666d..d5d8645cd4 100644
--- a/thunk.c
+++ b/thunk.c
@@ -67,7 +67,6 @@ void thunk_register_struct(int id, const char *name, const argtype *types)
     int nb_fields, offset, max_align, align, size, i, j;
 
     assert(id < max_struct_entries);
-    se = struct_entries + id;
 
     /* first we count the number of fields */
     type_ptr = types;
@@ -76,6 +75,8 @@ void thunk_register_struct(int id, const char *name, const argtype *types)
         type_ptr = thunk_type_next(type_ptr);
         nb_fields++;
     }
+    assert(nb_fields > 0);
+    se = struct_entries + id;
     se->field_types = types;
     se->nb_fields = nb_fields;
     se->name = name;