diff --git a/site-cookbooks/kosmos-mastodon/recipes/build.rb b/site-cookbooks/kosmos-mastodon/recipes/build.rb
index 3c85ccd..a849bd8 100644
--- a/site-cookbooks/kosmos-mastodon/recipes/build.rb
+++ b/site-cookbooks/kosmos-mastodon/recipes/build.rb
@@ -52,7 +52,8 @@ directory "#{mastodon_path}/.ssh" do
 end
 
 file "#{mastodon_path}/.ssh/id_ed25519" do
-  content credentials["repo_deploy_key"]
+  # OpenSSH's PEM parser rejects a private key without a trailing newline
+  content lazy { credentials["repo_deploy_key"].to_s.chomp + "\n" }
   owner mastodon_user
   group mastodon_user
   mode  "0600"