diff --git a/halo2-base/src/poseidon/hasher/tests/mod.rs b/halo2-base/src/poseidon/hasher/tests/mod.rs
index a734f7d0..76087be2 100644
--- a/halo2-base/src/poseidon/hasher/tests/mod.rs
+++ b/halo2-base/src/poseidon/hasher/tests/mod.rs
@@ -11,18 +11,18 @@ mod state;
 fn test_mds() {
     let spec = OptimizedPoseidonSpec::<Fr, 3, 2>::new::<8, 57, 0>();
 
-    let mds = vec![
-        vec![
+    let mds = [
+        [
             "7511745149465107256748700652201246547602992235352608707588321460060273774987",
             "10370080108974718697676803824769673834027675643658433702224577712625900127200",
             "19705173408229649878903981084052839426532978878058043055305024233888854471533",
         ],
-        vec![
+        [
             "18732019378264290557468133440468564866454307626475683536618613112504878618481",
             "20870176810702568768751421378473869562658540583882454726129544628203806653987",
             "7266061498423634438633389053804536045105766754026813321943009179476902321146",
         ],
-        vec![
+        [
             "9131299761947733513298312097611845208338517739621853568979632113419485819303",
             "10595341252162738537912664445405114076324478519622938027420701542910180337937",
             "11597556804922396090267472882856054602429588299176362916247939723151043581408",