diff --git a/main.c b/main.c
index 3507509..2d4223c 100644
--- a/main.c
+++ b/main.c
@@ -34,7 +34,7 @@
 #include <locale.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/main.c,v 1.225 2012/07/01 15:38:06 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/main.c,v 1.226 2012/07/22 15:56:50 tg Exp $");
 
 extern char **environ;
 
@@ -844,6 +844,7 @@ shell(Source * volatile s, volatile bool toplevel)
 }
 
 /* return to closest error handler or shell(), exit if none found */
+/* note: i MUST NOT be 0 */
 void
 unwind(int i)
 {
diff --git a/sh.h b/sh.h
index f6d79f7..3e6c8e8 100644
--- a/sh.h
+++ b/sh.h
@@ -157,7 +157,7 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.573 2012/07/20 23:22:13 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.574 2012/07/22 15:56:51 tg Exp $");
 #endif
 #define MKSH_VERSION "R40 2012/07/21"
 
@@ -631,6 +631,8 @@ enum sh_flag {
 
 /*
  * parsing & execution environment
+ *
+ * note that kshlongjmp MUST NOT be passed 0 as second argument!
  */
 #ifdef MKSH_NO_SIGSETJMP
 #define kshjmp_buf	jmp_buf
@@ -676,6 +678,7 @@ extern struct env {
 #define STOP_RETURN(t)	((t) == E_FUNC || (t) == E_INCL)
 
 /* values for kshlongjmp(e->jbuf, i) */
+/* note that i MUST NOT be zero */
 #define LRETURN	1	/* return statement */
 #define LEXIT	2	/* exit statement */
 #define LERROR	3	/* errorf() called */
@@ -1776,6 +1779,7 @@ int promptlen(const char *);
 int include(const char *, int, const char **, int);
 int command(const char *, int);
 int shell(Source * volatile, volatile bool);
+/* argument MUST NOT be 0 */
 void unwind(int) MKSH_A_NORETURN;
 void newenv(int);
 void quitenv(struct shf *);