Index: b/g10/build-packet.c
===================================================================
--- b/g10/build-packet.c
+++ b/g10/build-packet.c
@@ -1215,7 +1215,22 @@
 
 
 /****************
- * calculate the length of a header
+ * calculate the length of a header.
+ *
+ * LEN is the length of the packet's body.  NEW_CTB is whether we are
+ * using a new or old format packet.
+ *
+ * If LEN is 0 and NEW_CTB is 0, then we return a length that is
+ * appropriate for a body of indeterminate length.
+ *
+ * Note: it is not possible to compute the length of the header for
+ * packets whose body's length is 0 (this is not illegal, this is
+ * simply a limitation of the implementation).
+ *
+ * Note: if LEN is 0 and NEW_CTB is 1, this function will return 1,
+ * however, this is incorrect!  Thus, do not call this function with
+ * these parameters.  (Currently, no caller does this.  So, if you
+ * need this functionality, first fix this implementation.)
  */
 static int
 calc_header_length( u32 len, int new_ctb )