diff --git a/tests/_ostree_build.pm b/tests/_ostree_build.pm
index 8d17ab53..5a518bcd 100644
--- a/tests/_ostree_build.pm
+++ b/tests/_ostree_build.pm
@@ -74,7 +74,11 @@ sub run {
     # disables updating the ref with the new commit, and we *do* want
     # to do that. pungi updates the ref itself, I don't want to copy
     # all that work in here
-    assert_script_run "rpm-ostree compose tree --unified-core --repo=/var/tmp/ostree/repo/ --add-metadata-string=version=${advortask} --force-nocache /workstation-ostree-config/fedora-$lcsubv.yaml |& ts '" . '[%Y-%m-%d %H:%M:%S]' . "' | tee /tmp/ostree.log", 4500;
+    # we use --unified-core on F39+ as that's where it was turned on
+    # in prod
+    my $ucarg = "";
+    $ucarg = "--unified-core " if ($version > 38);
+    assert_script_run "rpm-ostree compose tree ${ucarg}--repo=/var/tmp/ostree/repo/ --add-metadata-string=version=${advortask} --force-nocache /workstation-ostree-config/fedora-$lcsubv.yaml |& ts '" . '[%Y-%m-%d %H:%M:%S]' . "' | tee /tmp/ostree.log", 4500;
     assert_script_run "set +o pipefail";
     upload_logs "/tmp/ostree.log";
     # check out the ostree installer lorax templates