diff --git a/ChangeLog b/ChangeLog
index 5b03e712b6..c5fcc2fbb4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Aug  6 20:12:28 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* rational.c (nurat_div): divided by float zero should be
+	  infinity.  [ruby-core:31626]
+
 Fri Aug  6 18:59:23 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* file.c (realpath_rec): rb_str_modify depends on the length, so
diff --git a/rational.c b/rational.c
index 6e3b39639d..d732f8d8a7 100644
--- a/rational.c
+++ b/rational.c
@@ -882,7 +882,7 @@ nurat_div(VALUE self, VALUE other)
 		    return DBL2NUM(-INFINITY);
 		}
 	    }
-	    if (modf(x, &den) == 0.0) {
+	    if (x != 0.0 && modf(x, &den) == 0.0) {
 		return rb_rational_raw2(dat->num, f_mul(rb_dbl2big(den), dat->den));
 	    }
 	}
diff --git a/test/ruby/test_rational.rb b/test/ruby/test_rational.rb
index 2f33ec2039..4680a79716 100644
--- a/test/ruby/test_rational.rb
+++ b/test/ruby/test_rational.rb
@@ -297,6 +297,8 @@ class Rational_Test < Test::Unit::TestCase
 
     assert_raise(ZeroDivisionError){Rational(1, 3) / 0}
     assert_raise(ZeroDivisionError){Rational(1, 3) / Rational(0)}
+
+    assert((Rational(1, 3) / 0.0).infinite?, '[ruby-core:31626]')
   end
 
   def assert_eql(exp, act, *args)
@@ -546,6 +548,7 @@ class Rational_Test < Test::Unit::TestCase
 
     assert_equal(0.25, c.fdiv(2))
     assert_equal(0.25, c.fdiv(2.0))
+    assert(c.fdiv(0).infinite?, '[ruby-core:31626]')
   end
 
   def test_expt
@@ -1067,6 +1070,7 @@ class Rational_Test < Test::Unit::TestCase
     assert_equal(Rational(5000000000), 10000000000.quo(2))
     assert_equal(0.5, 1.0.quo(2))
     assert_equal(Rational(1,4), Rational(1,2).quo(2))
+    assert(Rational(1,2).quo(0.0).infinite?, '[ruby-core:31626]')
 
     assert_equal(0.5, 1.fdiv(2))
     assert_equal(5000000000.0, 10000000000.fdiv(2))