diff --git a/skeletons/per_support.h b/skeletons/per_support.h
index 653c351..7cb1a0c 100644
--- a/skeletons/per_support.h
+++ b/skeletons/per_support.h
@@ -81,6 +81,9 @@
  */
 ssize_t uper_get_nsnnwn(asn_per_data_t *pd);
 
+/* Non-thread-safe debugging function, don't use it */
+char *per_data_string(asn_per_data_t *pd);
+
 /*
  * This structure supports forming PER output.
  */