guard PER references
diff --git a/skeletons/NULL.c b/skeletons/NULL.c
index 62bc91e..fb21c75 100644
--- a/skeletons/NULL.c
+++ b/skeletons/NULL.c
@@ -23,8 +23,13 @@
NULL_encode_der, /* Special handling of DER encoding */
NULL_decode_xer,
NULL_encode_xer,
+#ifdef ASN_DISABLE_PER_SUPPORT
+ 0,
+ 0,
+#else
NULL_decode_uper, /* Unaligned PER decoder */
NULL_encode_uper, /* Unaligned PER encoder */
+#endif /* ASN_DISABLE_PER_SUPPORT */
0, /* Use generic outmost tag fetcher */
asn_DEF_NULL_tags,
sizeof(asn_DEF_NULL_tags) / sizeof(asn_DEF_NULL_tags[0]),