diff --git a/skeletons/per_opentype.h b/skeletons/per_opentype.h
index 4e3f5fe..31c1832 100644
--- a/skeletons/per_opentype.h
+++ b/skeletons/per_opentype.h
@@ -22,7 +22,7 @@
  * Returns -1 if error is encountered. 0 if all OK.
  */
 int uper_open_type_put(asn_TYPE_descriptor_t *td,
-                       asn_per_constraints_t *constraints, void *sptr,
+                       const asn_per_constraints_t *constraints, void *sptr,
                        asn_per_outp_t *po);
 
 #ifdef __cplusplus