diff --git a/lib/redmine/scm/adapters/mercurial_adapter.rb b/lib/redmine/scm/adapters/mercurial_adapter.rb
index be755419..7607dd1c 100644
--- a/lib/redmine/scm/adapters/mercurial_adapter.rb
+++ b/lib/redmine/scm/adapters/mercurial_adapter.rb
@@ -153,15 +153,13 @@ module Redmine
         
         def diff(path, identifier_from, identifier_to=nil)
           path ||= ''
+          diff_args = ''
           if identifier_to
-            identifier_to = identifier_to.to_i 
+            diff_args = "-r #{hgrev(identifier_to)} -r #{hgrev(identifier_from)}"
           else
-            identifier_to = identifier_from.to_i - 1
+            diff_args = "-c #{hgrev(identifier_from)}"
           end
-          if identifier_from
-            identifier_from = identifier_from.to_i
-          end
-          cmd = "#{HG_BIN} -R #{target('')} diff -r #{identifier_to} -r #{identifier_from} --nodates"
+          cmd = "#{HG_BIN} -R #{target('')} diff --nodates #{diff_args}"
           cmd << " -I #{target(path)}" unless path.empty?
           diff = []
           shellout(cmd) do |io|
@@ -203,6 +201,12 @@ module Redmine
           return nil if $? && $?.exitstatus != 0
           blame
         end
+
+        # Returns correct revision identifier
+        def hgrev(identifier)
+          identifier.blank? ? 'tip' : identifier.to_s
+        end
+        private :hgrev
       end
     end
   end
diff --git a/test/unit/lib/redmine/scm/adapters/mercurial_adapter_test.rb b/test/unit/lib/redmine/scm/adapters/mercurial_adapter_test.rb
index afd7b270..088664bd 100644
--- a/test/unit/lib/redmine/scm/adapters/mercurial_adapter_test.rb
+++ b/test/unit/lib/redmine/scm/adapters/mercurial_adapter_test.rb
@@ -42,6 +42,24 @@ begin
         end
       end
 
+      def test_diff
+        assert_nil @adapter.diff(nil, '100000')
+        assert_nil @adapter.diff(nil, '100000', '200000')
+        [2, '400bb8672109', '400', 400].each do |r1|
+          diff1 = @adapter.diff(nil, r1)
+          assert_equal 28, diff1.size
+          assert_equal "+    return true unless klass.respond_to?('watched_by')\r\n", diff1[24]
+          [4, 'def6d2f1254a'].each do |r2|
+            diff2 = @adapter.diff(nil,r1,r2)
+            assert_equal 50, diff2.size
+            assert_equal "+class WelcomeController < ApplicationController\r\n", diff2[42]
+            diff3 = @adapter.diff('sources/watchers_controller.rb', r1, r2)
+            assert_equal 20, diff3.size
+            assert_equal "+    @watched.remove_watcher(user)\r\n", diff3[12]
+          end
+        end
+      end
+
       def test_cat
         [2, '400bb8672109', '400', 400].each do |r|
           buf = @adapter.cat('sources/welcome_controller.rb', r)