diff --git a/runbot/runbot.py b/runbot/runbot.py
index 81835744..f36e7aa0 100644
--- a/runbot/runbot.py
+++ b/runbot/runbot.py
@@ -670,7 +670,7 @@ class runbot_build(osv.osv):
             """, [repo.id, target_id])
             for common_name, in cr.fetchall():
                 try:
-                    commit = repo.git(['merge-base', branch.name, common_name]).strip()
+                    commit = repo.git(['merge-base', branch['name'], common_name]).strip()
                     cmd = ['log', '-1', '--format=%cd', '--date=iso', commit]
                     common_refs[common_name] = repo.git(cmd).strip()
                 except subprocess.CalledProcessError: