Index: app/models/issue.rb
===================================================================
--- app/models/issue.rb	(revision 17140)
+++ app/models/issue.rb	(working copy)
@@ -440,7 +440,7 @@
   end
 
   def estimated_hours=(h)
-    write_attribute :estimated_hours, (h.is_a?(String) ? h.to_hours : h)
+    write_attribute :estimated_hours, (h.is_a?(String) ? (h.to_hours || h) : h)
   end
 
   safe_attributes 'project_id',
Index: test/unit/issue_test.rb
===================================================================
--- test/unit/issue_test.rb	(revision 17140)
+++ test/unit/issue_test.rb	(working copy)
@@ -128,7 +128,7 @@
 
   def test_estimated_hours_should_be_validated
     set_language_if_valid 'en'
-    ['-2'].each do |invalid|
+    ['-2', '123abc'].each do |invalid|
       issue = Issue.new(:estimated_hours => invalid)
       assert !issue.valid?
       assert_include 'Estimated time is invalid', issue.errors.full_messages