diff --git a/src/update/index_documents/mod.rs b/src/update/index_documents/mod.rs
index ce4e8c2df..8860c1a93 100644
--- a/src/update/index_documents/mod.rs
+++ b/src/update/index_documents/mod.rs
@@ -697,17 +697,17 @@ mod tests {
         assert_eq!(count, 3);
 
         let docs = index.documents(&rtxn, vec![0, 1, 2]).unwrap();
-        let (kevin_id, _) = docs.iter().find(|(_, d)| d.get(1).unwrap() == br#""kevin""#).unwrap();
+        let (kevin_id, _) = docs.iter().find(|(_, d)| {
+            d.get(0).unwrap() == br#""updated kevin""#
+        }).unwrap();
         let (id, doc) = docs[*kevin_id as usize];
         assert_eq!(id, *kevin_id);
 
         // Check that this document is equal to the last
         // one sent and that an UUID has been generated.
-        let mut doc_iter = doc.iter();
+        assert_eq!(doc.get(0), Some(&br#""updated kevin""#[..]));
         // This is an UUID, it must be 36 bytes long plus the 2 surrounding string quotes (").
-        doc_iter.next().filter(|(_, id)| id.len() == 36 + 2).unwrap();
-        assert_eq!(doc_iter.next(), Some((1, &br#""kevin""#[..])));
-        assert_eq!(doc_iter.next(), None);
+        assert!(doc.get(1).unwrap().len() == 36 + 2);
         drop(rtxn);
     }
 
@@ -842,4 +842,36 @@ mod tests {
         assert_eq!(count, 3);
         drop(rtxn);
     }
+
+    #[test]
+    fn invalid_documents_ids() {
+        let path = tempfile::tempdir().unwrap();
+        let mut options = EnvOpenOptions::new();
+        options.map_size(10 * 1024 * 1024); // 10 MB
+        let index = Index::new(options, &path).unwrap();
+
+        // First we send 1 document with an invalid id.
+        let mut wtxn = index.write_txn().unwrap();
+        // There is a space in the document id.
+        let content = &b"id,name\nbrume bleue,kevin\n"[..];
+        let mut builder = IndexDocuments::new(&mut wtxn, &index);
+        builder.update_format(UpdateFormat::Csv);
+        assert!(builder.execute(content, |_, _| ()).is_err());
+        wtxn.commit().unwrap();
+
+        // First we send 1 document with a valid id.
+        let mut wtxn = index.write_txn().unwrap();
+        // There is a space in the document id.
+        let content = &b"id,name\n32,kevin\n"[..];
+        let mut builder = IndexDocuments::new(&mut wtxn, &index);
+        builder.update_format(UpdateFormat::Csv);
+        builder.execute(content, |_, _| ()).unwrap();
+        wtxn.commit().unwrap();
+
+        // Check that there is 1 document now.
+        let rtxn = index.read_txn().unwrap();
+        let count = index.number_of_documents(&rtxn).unwrap();
+        assert_eq!(count, 1);
+        drop(rtxn);
+    }
 }
diff --git a/src/update/index_documents/transform.rs b/src/update/index_documents/transform.rs
index df0dccc3f..ff71928cb 100644
--- a/src/update/index_documents/transform.rs
+++ b/src/update/index_documents/transform.rs
@@ -172,6 +172,12 @@ impl Transform<'_, '_> {
                     writer.insert(field_id, &json_buffer)?;
                 }
                 else if field_id == primary_key {
+                    // We validate the document id [a-zA-Z0-9\-_].
+                    let user_id = match validate_document_id(&user_id) {
+                        Some(valid) => valid,
+                        None => return Err(anyhow!("invalid document id: {:?}", user_id)),
+                    };
+
                     // We serialize the document id.
                     serde_json::to_writer(&mut json_buffer, &user_id)?;
                     writer.insert(field_id, &json_buffer)?;
@@ -256,9 +262,15 @@ impl Transform<'_, '_> {
             let mut writer = obkv::KvWriter::new(&mut obkv_buffer);
 
             // We extract the user id if we know where it is or generate an UUID V4 otherwise.
-            // TODO we must validate the user id (i.e. [a-zA-Z0-9\-_]).
             let user_id = match user_id_pos {
-                Some(pos) => &record[pos],
+                Some(pos) => {
+                    let user_id = &record[pos];
+                    // We validate the document id [a-zA-Z0-9\-_].
+                    match validate_document_id(&user_id) {
+                        Some(valid) => valid,
+                        None => return Err(anyhow!("invalid document id: {:?}", user_id)),
+                    }
+                },
                 None => uuid::Uuid::new_v4().to_hyphenated().encode_lower(&mut uuid_buffer),
             };
 
@@ -411,3 +423,12 @@ fn merge_obkvs(_key: &[u8], obkvs: &[Cow<[u8]>]) -> anyhow::Result<Vec<u8>> {
         buffer
     }))
 }
+
+fn validate_document_id(document_id: &str) -> Option<&str> {
+    let document_id = document_id.trim();
+    Some(document_id).filter(|id| {
+        !id.is_empty() && id.chars().all(|c| {
+            matches!(c, 'a'..='z' | 'A'..='Z' | '0'..='9' | '-' | '_')
+        })
+    })
+}