diff --git a/crates/engine/src/session_settings.rs b/crates/engine/src/session_settings.rs
index ef3e748a..0b457926 100644
--- a/crates/engine/src/session_settings.rs
+++ b/crates/engine/src/session_settings.rs
@@ -116,6 +116,11 @@ impl Session {
             self.layer_cache.set_disk(None);
             return;
         };
+        // Tests: the Roto/Object Matte segment store below is one per process, so a test that
+        // opens a disk cache (often the platform folder, through a settings store) would take
+        // over the store of a roto test running beside it. Serialise them with the roto tests.
+        #[cfg(test)]
+        crate::tests_roto::hold_roto_cache_test_lock();
         let dc = match &self.disk_cache {
             Some(c) if c.folder() == folder.as_path() => {
                 c.set_max_bytes(max);