diff --git a/libctf/ChangeLog b/libctf/ChangeLog
index 909c4fc87b5..a9e76ecd7ca 100644
--- a/libctf/ChangeLog
+++ b/libctf/ChangeLog
@@ -1,3 +1,9 @@
+2021-03-25  Nick Alcock  <nick.alcock@oracle.com>
+
+	PR libctf/27628
+	* ctf-serialize.c (ctf_emit_type_sect): Allow for a NULL vlen in
+	CTF_K_FUNCTION types.
+
 2021-03-25  Nick Alcock  <nick.alcock@oracle.com>
 
 	* ctf-dump.c (ctf_dump_format_type): Don't emit size or alignment
diff --git a/libctf/ctf-serialize.c b/libctf/ctf-serialize.c
index 0811b7b6efe..9f50280176c 100644
--- a/libctf/ctf-serialize.c
+++ b/libctf/ctf-serialize.c
@@ -849,7 +849,9 @@ ctf_emit_type_sect (ctf_dict_t *fp, unsigned char **tptr)
 	  break;
 
 	case CTF_K_FUNCTION:
-	  memcpy (t, dtd->dtd_vlen, sizeof (uint32_t) * (vlen + (vlen & 1)));
+	  /* Functions with no args also have no vlen.  */
+	  if (dtd->dtd_vlen)
+	    memcpy (t, dtd->dtd_vlen, sizeof (uint32_t) * (vlen + (vlen & 1)));
 	  t += sizeof (uint32_t) * (vlen + (vlen & 1));
 	  break;