diff --git a/ChangeLog b/ChangeLog
index a093f5aced..bc577b284e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Sep 21 21:52:41 2008  Tadayoshi Funaba  <tadf@dotrb.org>
+
+	* math.c (math_log): should check argc.
+
 Sun Sep 21 21:20:24 2008  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* complex.c: added two macros.
diff --git a/math.c b/math.c
index ac0d4c1fe7..340014b624 100644
--- a/math.c
+++ b/math.c
@@ -353,7 +353,7 @@ math_log(int argc, VALUE *argv)
     Need_Float(x);
     errno = 0;
     d = log(RFLOAT_VALUE(x));
-    if (!NIL_P(base)) {
+    if (argc == 2) {
 	Need_Float(base);
 	d /= log(RFLOAT_VALUE(base));
     }
diff --git a/test/ruby/test_math.rb b/test/ruby/test_math.rb
index 55f354664c..d4dbf9dbf0 100644
--- a/test/ruby/test_math.rb
+++ b/test/ruby/test_math.rb
@@ -113,6 +113,7 @@ class TestMath < Test::Unit::TestCase
     assert_equal(1.0/0, Math.log(1.0/0))
     assert_raise(Errno::EDOM, Errno::ERANGE) { Math.log(0) }
     assert_raise(Errno::EDOM, Errno::ERANGE) { Math.log(-1) }
+    assert_raise(TypeError) { Math.log(1,nil) }
   end
 
   def test_log2