diff --git a/ChangeLog b/ChangeLog
index 2ef06c2aba..2a564b23e0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Aug  5 19:59:55 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_set_len): bail out when buffer overflowed
+	  probably.
+
 Thu Aug  5 19:51:21 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* compile.c (iseq_compile_each): drop unused ARGSCAT results.
diff --git a/string.c b/string.c
index cb371c2662..0aa2e6c52c 100644
--- a/string.c
+++ b/string.c
@@ -1693,10 +1693,15 @@ rb_str_unlocktmp(VALUE str)
 void
 rb_str_set_len(VALUE str, long len)
 {
+    long capa;
+
     str_modifiable(str);
     if (STR_SHARED_P(str)) {
 	rb_raise(rb_eRuntimeError, "can't set length of shared string");
     }
+    if (len > (capa = (long)rb_str_capacity(str))) {
+	rb_bug("probable buffer overflow: %ld for %ld", len, capa);
+    }
     STR_SET_LEN(str, len);
     RSTRING_PTR(str)[len] = '\0';
 }