diff --git a/ChangeLog b/ChangeLog
index 7ef4fef868..43365c401f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Tue Jul  9 00:46:22 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (biglsh_bang): Fix bignum digits under-run.
+
 Mon Jul  8 23:36:45 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/dl/win32/lib/win32/registry.rb (Error, API): use WCHAR
diff --git a/bignum.c b/bignum.c
index 6e2758bae6..c93061d44f 100644
--- a/bignum.c
+++ b/bignum.c
@@ -4425,11 +4425,11 @@ biglsh_bang(BDIGIT *xds, long xn, unsigned long shift)
     zds = xds + xn - 1;
     xn -= s1 + 1;
     num = BIGLO(xds[xn]<<s2);
-    do {
+    while (0 < xn) {
 	*zds-- = num | xds[--xn]>>s3;
 	num = BIGLO(xds[xn]<<s2);
     }
-    while (xn > 0);
+    assert(xds <= zds);
     *zds = num;
     for (i = s1; i > 0; --i)
 	*zds-- = 0;