diff --git a/ChangeLog b/ChangeLog
index 07b188317b..0c4314ba59 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Oct 17 17:30:06 2009  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* bignum.c (big_split): fix off-by-one error.  [ruby-dev:39501]
+
 Sat Oct 17 16:34:27 2009  Tanaka Akira  <akr@fsij.org>
 
 	* parse.y (parser_yylex): fix token even after trailing under score.
diff --git a/bignum.c b/bignum.c
index 44775ff7f1..df66d33667 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1848,13 +1848,13 @@ big_split(VALUE v, long n, volatile VALUE *ph, volatile VALUE *pl)
 
     while (--hn && !vds[hn + ln]);
     h = bignew(hn += 2, 1);
-    MEMCPY(BDIGITS(h), vds + ln, BDIGIT, hn);
-    BDIGITS(h)[hn - 1] = 0;
+    MEMCPY(BDIGITS(h), vds + ln, BDIGIT, hn - 1);
+    BDIGITS(h)[hn - 1] = 0; /* margin for carry */
 
     while (--ln && !vds[ln]);
     l = bignew(ln += 2, 1);
-    MEMCPY(BDIGITS(l), vds, BDIGIT, ln);
-    BDIGITS(l)[ln - 1] = 0;
+    MEMCPY(BDIGITS(l), vds, BDIGIT, ln - 1);
+    BDIGITS(l)[ln - 1] = 0; /* margin for carry */
 
     *pl = l;
     *ph = h;