diff --git a/ChangeLog b/ChangeLog
index 96c230e510..403c0852e8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Sep  4 20:22:43 2013  Tanaka Akira  <akr@fsij.org>
+
+	* bignum.c (bigdivrem_normal): Add assertions.
+
 Wed Sep  4 19:18:40 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* internal.h (vm_state_version_t): prefer LONG_LONG to uint64_t.
diff --git a/bignum.c b/bignum.c
index 039b56c597..ab702082b2 100644
--- a/bignum.c
+++ b/bignum.c
@@ -2640,6 +2640,9 @@ bigdivrem_normal(BDIGIT *zds, size_t zn, const BDIGIT *xds, size_t xn, BDIGIT *y
 {
     int shift;
 
+    assert(zn == xn + 1);
+    assert(yn < xn || (xn == yn && yds[yn - 1] <= xds[xn - 1]));
+
     shift = nlz(yds[yn-1]);
     if (shift) {
         bary_small_lshift(yds, yds, yn, shift);