diff --git a/roles/prometheus/tasks/prometheus.yml b/roles/prometheus/tasks/prometheus.yml
index 388014a..df31920 100644
--- a/roles/prometheus/tasks/prometheus.yml
+++ b/roles/prometheus/tasks/prometheus.yml
@@ -21,7 +21,7 @@
     mode: 0644
     validate: /usr/bin/promtool check rules %s
   vars:
-    alert_groups: "{{ hostvars[inventory_hostname] | dict2items | selectattr('key', 'match', '^prometheus_job_.+$') | map(attribute='value') | map(attribute='group') | unique | default([prometheus_job_default_group_name]) }}"
+    alert_groups: "{{ hostvars[inventory_hostname] | dict2items | selectattr('key', 'match', '^prometheus_job_.+$') | map(attribute='value') | map(attribute='group') | select('defined') | unique | default([prometheus_job_default_group_name]) }}"
     jobs: "{{ hostvars[inventory_hostname] | dict2items | selectattr('key', 'match', '^prometheus_job_.+$') | map(attribute='value') | list }}"
   notify: restart prometheus