diff --git a/addons/project/project.py b/addons/project/project.py
index 72a7958e921..cc885eb5d95 100644
--- a/addons/project/project.py
+++ b/addons/project/project.py
@@ -433,8 +433,9 @@ class project_work(osv.osv):
     def write(self, cr, uid, ids,vals,context={}):
         if 'hours' in vals and (not vals['hours']):
             vals['hours'] = 0.00
-        for work in self.browse(cr, uid, ids, context):
-            cr.execute('update project_task set remaining_hours=remaining_hours - %s + (%s) where id=%s', (vals.get('hours',0.0), work.hours, work.task_id.id))
+        if 'hours' in vals:
+            for work in self.browse(cr, uid, ids, context):
+                cr.execute('update project_task set remaining_hours=remaining_hours - %s + (%s) where id=%s', (vals.get('hours',0.0), work.hours, work.task_id.id))
         return super(project_work,self).write(cr, uid, ids, vals, context)
 
     def unlink(self, cr, uid, ids, *args, **kwargs):