diff --git a/app/models/issue.rb b/app/models/issue.rb
index fec0568..0521bc0 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1355,7 +1355,7 @@ class Issue < ActiveRecord::Base
       unless Issue.use_status_for_done_ratio? && p.status && p.status.default_done_ratio
         leaves_count = p.leaves.count
         if leaves_count > 0
-          average = p.leaves.average(:estimated_hours).to_f
+          average = p.leaves.average("CASE WHEN estimated_hours > 0 THEN estimated_hours ELSE NULL END").to_f
           if average == 0
             average = 1
           end
diff --git a/test/unit/issue_nested_set_test.rb b/test/unit/issue_nested_set_test.rb
index 9c4060b..adc989b 100644
--- a/test/unit/issue_nested_set_test.rb
+++ b/test/unit/issue_nested_set_test.rb
@@ -338,11 +338,14 @@ class IssueNestedSetTest < ActiveSupport::TestCase
   def test_parent_done_ratio_with_child_estimate_to_0_should_reach_100
     parent = Issue.generate!
     issue1 = Issue.generate!(:parent_issue_id => parent.id)
-    issue2 = Issue.generate!(:parent_issue_id => parent.id, :estimated_hours => 0)
+    issue2 = Issue.generate!(:parent_issue_id => parent.id, :estimated_hours => 10)
+    issue3 = Issue.generate!(:parent_issue_id => parent.id, :estimated_hours => 0)
     assert_equal 0, parent.reload.done_ratio
     issue1.reload.update_attribute :status_id, 5
-    assert_equal 50, parent.reload.done_ratio
+    assert_equal 33, parent.reload.done_ratio
     issue2.reload.update_attribute :status_id, 5
+    assert_equal 67, parent.reload.done_ratio
+    issue3.reload.update_attribute :status_id, 5
     assert_equal 100, parent.reload.done_ratio
   end