diff --git a/Cargo.lock b/Cargo.lock
index 66b5e0147..a1608dbd6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -611,7 +611,7 @@ checksum = "9b919933a397b79c37e33b77bb2aa3dc8eb6e165ad809e58ff75bc7db2e34574"
 [[package]]
 name = "grenad"
 version = "0.1.0"
-source = "git+https://github.com/Kerollmops/grenad.git?rev=c390cfe#c390cfed1dc8a26ca108ffaeb7bdd978fa4e9021"
+source = "git+https://github.com/Kerollmops/grenad.git?rev=ce3517f#ce3517fdbcf7ff0e1e703a4abbc623f69f29d8e0"
 dependencies = [
  "byteorder",
  "flate2",
diff --git a/Cargo.toml b/Cargo.toml
index 264cddc50..0a15aa653 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,7 +13,7 @@ csv = "1.1.3"
 flate2 = "1.0.17"
 fst = "0.4.4"
 fxhash = "0.2.1"
-grenad = { git = "https://github.com/Kerollmops/grenad.git", rev = "c390cfe" }
+grenad = { git = "https://github.com/Kerollmops/grenad.git", rev = "ce3517f" }
 heed = { version = "0.8.1", default-features = false, features = ["lmdb"] }
 human_format = "1.0.3"
 indexmap = { version = "1.6.0", features = ["serde-1"] }
diff --git a/src/indexing/transform.rs b/src/indexing/transform.rs
index 40d34a1b7..f5187c2df 100644
--- a/src/indexing/transform.rs
+++ b/src/indexing/transform.rs
@@ -1,12 +1,12 @@
 use std::borrow::Cow;
-use std::convert::{TryFrom, TryInto};
+use std::convert::TryFrom;
 use std::fs::File;
-use std::io::{Read, Write, Seek, SeekFrom};
+use std::io::{Read, Seek, SeekFrom};
 
-use anyhow::{bail, Context};
+use anyhow::Context;
 use crate::{FieldsIdsMap, AvailableDocumentsIds};
 use fst::{IntoStreamer, Streamer};
-use grenad::{Writer, Sorter, Reader, CompressionType};
+use grenad::{Writer, Sorter, CompressionType};
 use roaring::RoaringBitmap;
 
 pub struct TransformOutput {
@@ -49,7 +49,7 @@ impl<A: AsRef<[u8]>> Transform<A> {
 
         /// The last value associated with an id is kept.
         fn merge_last_win(_key: &[u8], vals: &[Cow<[u8]>]) -> anyhow::Result<Vec<u8>> {
-            Ok(vals.last().unwrap().clone().into_owned())
+            vals.last().context("no last value").map(|last| last.clone().into_owned())
         }
 
         // We initialize the sorter with the user indexing settings.
@@ -63,23 +63,27 @@ impl<A: AsRef<[u8]>> Transform<A> {
         // We write into the sorter to merge and deduplicate the documents
         // based on the users ids.
         let mut sorter = sorter_builder.build();
-        let mut buffer = Vec::new();
+        let mut json_buffer = Vec::new();
+        let mut obkv_buffer = Vec::new();
         let mut record = csv::StringRecord::new();
         while csv.read_record(&mut record)? {
 
-            buffer.clear();
-            let mut writer = obkv::KvWriter::new(&mut buffer);
+            obkv_buffer.clear();
+            let mut writer = obkv::KvWriter::new(&mut obkv_buffer);
 
             // We retrieve the field id based on the CSV header position
             // and zip it with the record value.
             for (key, field) in fields_ids.iter().copied().zip(&record) {
-                // TODO we must serialize the values as JSON strings.
-                writer.insert(key, field)?;
+                // We serialize the attribute values as JSON strings.
+                json_buffer.clear();
+                serde_json::to_writer(&mut json_buffer, &field)?;
+                writer.insert(key, &json_buffer)?;
             }
 
             // We extract the user id and use it as the key for this document.
+            // TODO we must validate the user id (i.e. [a-zA-Z0-9\-_]).
             let user_id = &record[user_id_pos];
-            sorter.insert(user_id, &buffer)?;
+            sorter.insert(user_id, &obkv_buffer)?;
         }
 
         // Once we have sort and deduplicated the documents we write them into a final file.
@@ -129,7 +133,7 @@ impl<A: AsRef<[u8]>> Transform<A> {
 
         // We create the union between the existing users ids documents ids with the new ones.
         let new_users_ids_documents_ids = new_users_ids_documents_ids_builder.into_map();
-        let mut union_ = fst::map::OpBuilder::new()
+        let union_ = fst::map::OpBuilder::new()
             .add(&self.users_ids_documents_ids)
             .add(&new_users_ids_documents_ids)
             .r#union();