diff --git a/ChangeLog b/ChangeLog
index ab7cc04fe0..c56458ef28 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -14,6 +14,10 @@ Fri Aug  4 13:56:51 2006  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 	* pack.c (pack_pack): check argument overrun for 'P'.  based on a
 	  patch by rucila <rucila at yahoo.cojp>.  fixed: [ruby-dev:29182]
 
+Fri Aug  4 02:42:29 2006  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* sprintf.c (rb_str_format): a bug in %c type check.
+
 Fri Aug  4 01:28:19 2006  Tanaka Akira  <akr@fsij.org>
 
 	* io.c (io_reopen): STDERR.reopen(File.open("/dev/null", "w")) should
diff --git a/sprintf.c b/sprintf.c
index 953ce5054c..4c5e56843c 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -399,13 +399,15 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
 	  case 'c':
 	    {
 		VALUE val = GETARG();
+		VALUE tmp;
 		char c;
 
-		if (rb_check_string_type(val)) {
-		    if (RSTRING(val)->len != 1) {
+		tmp = rb_check_string_type(val);
+		if (!NIL_P(tmp)) {
+		    if (RSTRING(tmp)->len != 1) {
 			rb_raise(rb_eArgError, "%%c requires a character");
 		    }
-		    c = RSTRING(val)->ptr[0];
+		    c = RSTRING(tmp)->ptr[0];
 		}
 		else {
 		    c = NUM2INT(val) & 0xff;