diff --git a/mastodon.nix b/mastodon.nix
index facda0e..95fde42 100644
--- a/mastodon.nix
+++ b/mastodon.nix
@@ -32,7 +32,7 @@ in
       };
     };
     services.mastodon = {
-      extraConfig = {
+      extraConfig = rec {
         S3_ENABLED = "true";
         S3_ENDPOINT = "http://s3.garage.localhost:3900";
         S3_REGION = "garage";
@@ -44,8 +44,7 @@ in
         S3_PROTOCOL = "http";
         S3_HOSTNAME = "web.garage.localhost:3902";
         # by default it tries to use "<S3_HOSTNAME>/<S3_BUCKET>"
-        # but we want "<S3_BUCKET>.<S3_HOSTNAME>"
-        S3_ALIAS_HOST = "mastodon.web.garage.localhost:3902";
+        S3_ALIAS_HOST = "${S3_BUCKET}.${S3_HOSTNAME}";
         # SEE: the last section in https://docs.joinmastodon.org/admin/optional/object-storage/
         # TODO: can we set up ACLs with garage?
         S3_PERMISSION = "";
diff --git a/tests/mastodon-garage.nix b/tests/mastodon-garage.nix
index f40d1ce..ce04e91 100644
--- a/tests/mastodon-garage.nix
+++ b/tests/mastodon-garage.nix
@@ -113,15 +113,21 @@ rebuildableTest {
 
     with subtest("Content security policy allows garage images"):
       headers = server.succeed("xh -h http://masstodon.localhost:55001/public/local")
-      csp_match = re.match('^Content-Security-Policy: (.*)$', headers, re.M)
+      csp_match = None
+      # I can't figure out re.MULTILINE
+      for header in headers.split("\n"):
+        csp_match =  re.match('^Content-Security-Policy: (.*)$', header)
+        if csp_match is not None:
+          break
       if csp_match is None:
         raise Exception("mastodon did not send a content security policy header")
       csp = csp_match.group(1)
-      # the content security policy should include the garage server
-      garage_csp = re.match(".*web\.garage\.localhost:3902.*", csp)
+      # the img-src content security policy should include the garage server
+      garage_csp = re.match(".*; img-src[^;]*web\.garage\.localhost:3902.*", csp)
       if garage_csp is None:
         raise Exception("Mastodon's content security policy does not include garage server. image will not be displayed properly on mastodon.")
 
+    # this could in theory give a false positive if mastodon changes it's colorscheme to include pure green.
     with subtest("image displays"):
       server.succeed("selenium-script")
       server.copy_from_vm("/mastodon-screenshot.png", "")