diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e6d1c144813c75be6efd73737820df8ff04fd416..3ce4720491a26f54baf895e14ea1feb5fe3aa5da 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -102,4 +102,8 @@ armarx-core:
 
   trigger:
     project: sw/armarx/armarx-core
-    branch: master  # This is optional
+    # This is optional, but we could tell ArmarX which Simox branch to use.
+    branch: master  
+
+  rules:
+    - if: $CI_COMMIT_BRANCH == "master"