diff --git a/ChangeLog b/ChangeLog
index 98694cb1b8..6a96aba62d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Aug 18 10:05:12 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (rb_big2str1): Make an expression more explicit.
+
 Sun Aug 18 03:18:45 2013  Tanaka Akira  <akr@fsij.org>
 
 	* bignum.c (rb_big2str1): Use power_level instead of bitsize(xn).
diff --git a/bignum.c b/bignum.c
index 0da5199b59..37a0d5ebc9 100644
--- a/bignum.c
+++ b/bignum.c
@@ -4569,7 +4569,7 @@ rb_big2str1(VALUE x, int base)
         VALUE tmpw = 0;
         BDIGIT *wds;
         size_t wn;
-        wn = power_level + RBIGNUM_LEN(power);
+        wn = power_level * BIGDIVREM_EXTRA_WORDS + RBIGNUM_LEN(power);
         wds = ALLOCV_N(BDIGIT, tmpw, xn + wn);
         MEMCPY(wds, xds, BDIGIT, xn);
 	big2str_karatsuba(&b2s_data, wds, xn, wn, power_level, 0);