diff --git a/app/models/issue.rb b/app/models/issue.rb
index 8a0ef7f..9980303 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -67,7 +67,7 @@ class Issue < ActiveRecord::Base
 
   validates_length_of :subject, :maximum => 255
   validates_inclusion_of :done_ratio, :in => 0..100
-  validates_numericality_of :estimated_hours, :allow_nil => true
+  validates :estimated_hours, :numericality => {:greater_than_or_equal_to => 0, :allow_nil => true, :message => :invalid}
   validates :start_date, :date => true
   validates :due_date, :date => true
   validate :validate_issue, :validate_required_fields
diff --git a/test/unit/issue_test.rb b/test/unit/issue_test.rb
index 1ef94c6..cb89d37 100644
--- a/test/unit/issue_test.rb
+++ b/test/unit/issue_test.rb
@@ -79,6 +79,15 @@ class IssueTest < ActiveSupport::TestCase
     assert_include 'Due date must be greater than start date', issue.errors.full_messages
   end
 
+  def test_estimated_hours_should_be_validated
+    set_language_if_valid 'en'
+    ['-2'].each do |invalid|
+      issue = Issue.new(:estimated_hours => invalid)
+      assert !issue.valid?
+      assert_include 'Estimated time is invalid', issue.errors.full_messages
+    end
+  end
+
   def test_create_with_required_custom_field
     set_language_if_valid 'en'
     field = IssueCustomField.find_by_name('Database')