diff --git a/tests/pixelfed-garage.nix b/tests/pixelfed-garage.nix
index 21d4b4b..b06c438 100644
--- a/tests/pixelfed-garage.nix
+++ b/tests/pixelfed-garage.nix
@@ -128,11 +128,14 @@ pkgs.nixosTest {
     # there, then post a green image and check that the green pixel IS there.
 
     with subtest("Image displays"):
-      server.succeed(f"su - selenium -c 'selenium-script test@test.com {password}'")
+      server.succeed(f"su - selenium -c 'selenium-script test@test.com {password}' >&2")
       server.copy_from_vm("/home/selenium/screenshot.png", "")
-      displayed_colors = server.succeed("convert /screenshot.png -define histogram:unique-colors=true -format %c histogram:info:")
+      displayed_colors = subprocess.run(
+          ["magick", "screenshot.png", "-define", "histogram:unique-colors=true", "-format", "%c", "histogram:info:"],
+          capture_output=True
+      )
       # check that the green image displayed somewhere
-      green_check = re.match(".*#00FF00.*", displayed_colors, re.S)
+      green_check = re.match(".*#00FF00.*", displayed_colors.stdout.decode(), re.S)
       if green_check is None:
         raise Exception("cannot detect the uploaded image on pixelfed page.")
   '';