diff --git a/ChangeLog b/ChangeLog
index 7eb0c30ab5..500e0e9b11 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Mar 20 22:51:46 2010  Tanaka Akira  <akr@fsij.org>
+
+	* string.c (rb_str_sum): use UCHAR_MAX.
+
 Sat Mar 20 19:16:42 2010  Tanaka Akira  <akr@fsij.org>
 
 	* tool/transcode-tblgen.rb (ActionMap.unambiguous_action): use
diff --git a/string.c b/string.c
index 29d083a230..1186b3b81b 100644
--- a/string.c
+++ b/string.c
@@ -6563,7 +6563,7 @@ rb_str_sum(int argc, VALUE *argv, VALUE str)
     pend = p + len;
 
     while (p < pend) {
-        if (FIXNUM_MAX - 255 < sum0) {
+        if (FIXNUM_MAX - UCHAR_MAX < sum0) {
             sum = rb_funcall(sum, '+', 1, LONG2FIX(sum0));
             str_mod_check(str, ptr, len);
             sum0 = 0;