diff --git a/runbot/runbot.py b/runbot/runbot.py
index 285cdb57..7fabca4f 100644
--- a/runbot/runbot.py
+++ b/runbot/runbot.py
@@ -637,10 +637,10 @@ class runbot_build(osv.osv):
 
             # build complete set of modules to install
             modules_to_move = []
-            modules_to_test = ((build.modules or '') +
-                               (build.branch_id.modules or '') +
+            modules_to_test = ((build.modules or '') + ',' +
+                               (build.branch_id.modules or '') + ',' +
                                (build.repo_id.modules or ''))
-            modules_to_test = modules_to_test.split(',') if modules_to_test else []
+            modules_to_test = filter(None, modules_to_test.split(','))
             _logger.debug("manual modules_to_test for build %s: %s", build.dest, modules_to_test)
 
             if not has_server: