diff --git a/ChangeLog b/ChangeLog
index da6066a240..c765976b87 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Dec 23 00:16:48 2008  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* sprintf.c (rb_str_format): fix buffer overflow.
+
 Mon Dec 22 19:31:19 2008  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
 
 	* common.mk (revision.h): uses tool/file2lastrev.rb to support
diff --git a/sprintf.c b/sprintf.c
index 1195f9b17b..cc8f097e5b 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -979,8 +979,8 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 		    if ((flags & FWIDTH) && need < width)
 			need = width;
 
-		    CHECK(need);
-		    snprintf(&buf[blen], need, "%*s", need, "");
+		    CHECK(need + 1);
+		    snprintf(&buf[blen], need + 1, "%*s", need, "");
 		    if (flags & FMINUS) {
 			if (!isnan(fval) && fval < 0.0)
 			    buf[blen++] = '-';