diff --git a/plonky2x/core/src/frontend/eth/mpt/builder.rs b/plonky2x/core/src/frontend/eth/mpt/builder.rs
index 5356e133..63b9cf96 100644
--- a/plonky2x/core/src/frontend/eth/mpt/builder.rs
+++ b/plonky2x/core/src/frontend/eth/mpt/builder.rs
@@ -256,7 +256,7 @@ mod tests {
         );
         let circuit = builder.mock_build();
 
-        let mut key_bytes = vec![];
+        let mut key_bytes = vec![0u8; 32];
         key.to_big_endian(&mut key_bytes);
 
         let key_bytes: [u8; 32] = key_bytes.try_into().unwrap();
diff --git a/plonky2x/core/src/frontend/eth/mpt/reference.rs b/plonky2x/core/src/frontend/eth/mpt/reference.rs
index 60a6b8eb..5f622163 100644
--- a/plonky2x/core/src/frontend/eth/mpt/reference.rs
+++ b/plonky2x/core/src/frontend/eth/mpt/reference.rs
@@ -159,7 +159,7 @@ mod tests {
             .map(|b| b.to_vec())
             .collect::<Vec<Vec<u8>>>();
 
-        let mut key_bytes = vec![];
+        let mut key_bytes = vec![0u8; 32];
         storage_result.storage_proof[0]
             .key
             .to_big_endian(&mut key_bytes);