diff --git a/g10/build-packet.c b/g10/build-packet.c
index 5cc03cf65..d7f2291f2 100644
--- a/g10/build-packet.c
+++ b/g10/build-packet.c
@@ -233,12 +233,17 @@ do_user_id( IOBUF out, int ctb, PKT_user_id *uid )
 
     if( uid->attrib_data )
       {
-	write_header(out, ctb, uid->attrib_len);
+        /* We need to take special care of a user ID with a length of 0:
+         * Without forcing HDRLEN to 2 in this case an indeterminate length
+         * packet would be written which is not allowed.  Note that we are
+         * always called with a CTB indicating an old packet header format,
+         * so that forcing a 2 octet header works.  */
+	write_header2 (out, ctb, uid->attrib_len, (uid->attrib_len? 0 : 2));
 	rc = iobuf_write( out, uid->attrib_data, uid->attrib_len );
       }
     else
       {
-        write_header2( out, ctb, uid->len, 2 );
+        write_header2 (out, ctb, uid->len, 2);
 	rc = iobuf_write( out, uid->name, uid->len );
       }
     return rc;