diff --git a/ChangeLog b/ChangeLog
index a28f914860..c000924a55 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Wed Mar  4 00:54:43 2009  Yuki Sonoda (Yugui)  <yugui@yugui.jp>
+
+	* lib/prime.rb (Prime::prime?): used to return a wrong answer.
+	  [ruby-core:22646].
+
+	* test/test_prime.rb (test_prime?): test case for [ruby-core:22646].
+
 Tue Mar  3 17:10:09 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/extmk.rb (extmake): removes object files no longer used and
diff --git a/lib/prime.rb b/lib/prime.rb
index 650d279bc4..ce71d5e00f 100644
--- a/lib/prime.rb
+++ b/lib/prime.rb
@@ -144,6 +144,8 @@ class Prime
   # +value+:: an arbitrary integer to be checked.
   # +generator+:: optional. A pseudo-prime generator.
   def prime?(value, generator = Prime::Generator23.new)
+    value = -value if value < 0
+    return false if value < 2
     for num in generator
       q,r = value.divmod num
       return true if q < num
diff --git a/test/test_prime.rb b/test/test_prime.rb
index aca5109689..0dad7cf987 100644
--- a/test/test_prime.rb
+++ b/test/test_prime.rb
@@ -89,6 +89,11 @@ class TestPrime < Test::Unit::TestCase
     end
   end
 
+  def test_default_instance_does_not_have_compatibility_methods
+    assert !Prime.instance.respond_to?(:succ)
+    assert !Prime.instance.respond_to?(:next)
+  end
+
   class TestInteger < Test::Unit::TestCase
     def test_prime_division
       pd = PRIMES.inject(&:*).prime_division
@@ -100,6 +105,10 @@ class TestPrime < Test::Unit::TestCase
     end
 
     def test_prime?
+      # zero and unit
+      assert !0.prime?
+      assert !1.prime?
+
       # small primes
       assert 2.prime?
       assert 3.prime?
@@ -121,6 +130,12 @@ class TestPrime < Test::Unit::TestCase
 
       # factorial
       assert !(2...100).inject(&:*).prime?
+
+      # negative
+      assert !-1.prime?
+      assert -2.prime?
+      assert -3.prime?
+      assert !-4.prime?
     end
   end
 end