diff --git a/check.t b/check.t
index 43c1238..13f26b7 100644
--- a/check.t
+++ b/check.t
@@ -1,4 +1,4 @@
-# $MirOS: src/bin/mksh/check.t,v 1.383 2010/07/19 22:41:01 tg Exp $
+# $MirOS: src/bin/mksh/check.t,v 1.384 2010/07/21 11:31:13 tg Exp $
 # $OpenBSD: bksl-nl.t,v 1.2 2001/01/28 23:04:56 niklas Exp $
 # $OpenBSD: history.t,v 1.5 2001/01/28 23:04:56 niklas Exp $
 # $OpenBSD: read.t,v 1.3 2003/03/10 03:48:16 david Exp $
@@ -25,7 +25,7 @@
 # http://www.research.att.com/~gsf/public/ifs.sh
 
 expected-stdout:
-	@(#)MIRBSD KSH R39 2010/07/19
+	@(#)MIRBSD KSH R39 2010/07/21
 description:
 	Check version of shell.
 stdin:
@@ -185,6 +185,17 @@ expected-stdout:
 	tf
 	tf
 ---
+name: alias-10
+description:
+	Check that recursion is detected/avoided in aliases.
+	Regression, introduced during an old bugfix.
+stdin:
+	alias foo='print hello '
+	alias bar='foo world'
+	echo $(bar)
+expected-stdout:
+	hello world
+---
 name: arith-lazy-1
 description:
 	Check that only one side of ternary operator is evaluated
diff --git a/lex.c b/lex.c
index e7814de..f242f2a 100644
--- a/lex.c
+++ b/lex.c
@@ -22,7 +22,7 @@
 
 #include "sh.h"
 
-__RCSID("$MirOS: src/bin/mksh/lex.c,v 1.116 2010/07/17 22:09:36 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/lex.c,v 1.117 2010/07/21 11:31:15 tg Exp $");
 
 /*
  * states while lexing word
@@ -1110,7 +1110,7 @@ yylex(int cf)
 			else {
 				Source *s = source;
 
-				while (s->flags & SF_HASALIAS)
+				while (s && (s->flags & SF_HASALIAS))
 					if (s->u.tblp == p)
 						return (LWORD);
 					else
diff --git a/sh.h b/sh.h
index 5d0a9eb..9c387eb 100644
--- a/sh.h
+++ b/sh.h
@@ -150,9 +150,9 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.399 2010/07/19 22:41:03 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.400 2010/07/21 11:31:16 tg Exp $");
 #endif
-#define MKSH_VERSION "R39 2010/07/19"
+#define MKSH_VERSION "R39 2010/07/21"
 
 #ifndef MKSH_INCLUDES_ONLY