diff --git a/crates/matrix-store-key/src/lib.rs b/crates/matrix-store-key/src/lib.rs
index b496b729d..fc6c372a6 100644
--- a/crates/matrix-store-key/src/lib.rs
+++ b/crates/matrix-store-key/src/lib.rs
@@ -348,6 +348,8 @@ impl StoreKey {
     }
 }
 
+#[derive(Zeroize)]
+#[zeroize(drop)]
 struct MacKey(Box<[u8; 32]>);
 
 impl MacKey {