diff --git a/configs/workhorse/prometheus.nix b/configs/workhorse/prometheus.nix
index 9a282ce..574c0ed 100644
--- a/configs/workhorse/prometheus.nix
+++ b/configs/workhorse/prometheus.nix
@@ -13,7 +13,7 @@
   services.prometheus = {
     enable = true;
     # keep data for 30 days
-    extraFlags = "--storage.tsdb.retention.time=30d";
+    extraFlags = ["--storage.tsdb.retention.time=30d"];
     scrapeConfigs = [
       {
         job_name = "nginx";