diff --git a/Cargo.lock b/Cargo.lock
index c3222c7fd..30b1102b5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2623,6 +2623,7 @@ dependencies = [
  "meilisearch-types",
  "memmap2",
  "page_size",
+ "raw-collections",
  "rayon",
  "roaring",
  "serde",
diff --git a/crates/index-scheduler/Cargo.toml b/crates/index-scheduler/Cargo.toml
index 4a2913083..deaded910 100644
--- a/crates/index-scheduler/Cargo.toml
+++ b/crates/index-scheduler/Cargo.toml
@@ -22,6 +22,7 @@ flate2 = "1.0.30"
 meilisearch-auth = { path = "../meilisearch-auth" }
 meilisearch-types = { path = "../meilisearch-types" }
 page_size = "0.6.0"
+raw-collections = { git = "https://github.com/dureuill/raw-collections.git", version = "0.1.0" }
 rayon = "1.10.0"
 roaring = { version = "0.10.6", features = ["serde"] }
 serde = { version = "1.0.204", features = ["derive"] }
diff --git a/crates/index-scheduler/src/batch.rs b/crates/index-scheduler/src/batch.rs
index bd307b19e..4ae8c7d46 100644
--- a/crates/index-scheduler/src/batch.rs
+++ b/crates/index-scheduler/src/batch.rs
@@ -43,6 +43,7 @@ use meilisearch_types::milli::{self, Filter};
 use meilisearch_types::settings::{apply_settings_to_builder, Settings, Unchecked};
 use meilisearch_types::tasks::{Details, IndexSwap, Kind, KindWithContent, Status, Task};
 use meilisearch_types::{compression, Index, VERSION_FILE_NAME};
+use raw_collections::RawMap;
 use roaring::RoaringBitmap;
 use time::macros::format_description;
 use time::OffsetDateTime;
@@ -1318,7 +1319,12 @@ impl IndexScheduler {
                     index,
                     &mut new_fields_ids_map,
                     primary_key.as_deref(),
-                    first_document.as_ref(),
+                    first_document
+                        .map(|raw| RawMap::from_raw_value(raw, &indexer_alloc))
+                        .transpose()
+                        .map_err(|error| {
+                            milli::Error::UserError(milli::UserError::SerdeJson(error))
+                        })?,
                 )?
                 .map_err(milli::Error::from)?;
 
diff --git a/crates/milli/src/update/new/indexer/mod.rs b/crates/milli/src/update/new/indexer/mod.rs
index 3b66c2ec0..ca61a9b7b 100644
--- a/crates/milli/src/update/new/indexer/mod.rs
+++ b/crates/milli/src/update/new/indexer/mod.rs
@@ -12,6 +12,7 @@ use heed::{RoTxn, RwTxn};
 use itertools::{merge_join_by, EitherOrBoth};
 pub use partial_dump::PartialDump;
 use rand::SeedableRng as _;
+use raw_collections::RawMap;
 use rayon::ThreadPool;
 use time::OffsetDateTime;
 pub use update_by_function::UpdateByFunction;
@@ -24,7 +25,7 @@ use super::word_fst_builder::{PrefixData, PrefixDelta, WordFstBuilder};
 use super::words_prefix_docids::{
     compute_word_prefix_docids, compute_word_prefix_fid_docids, compute_word_prefix_position_docids,
 };
-use super::{StdResult, TopLevelMap};
+use super::StdResult;
 use crate::documents::{PrimaryKey, DEFAULT_PRIMARY_KEY};
 use crate::facet::FacetType;
 use crate::fields_ids_map::metadata::{FieldIdMapWithMetadata, MetadataBuilder};
@@ -733,7 +734,7 @@ pub fn retrieve_or_guess_primary_key<'a>(
     index: &Index,
     new_fields_ids_map: &mut FieldsIdsMap,
     primary_key_from_op: Option<&'a str>,
-    first_document: Option<&'a TopLevelMap<'a>>,
+    first_document: Option<RawMap<'a>>,
 ) -> Result<StdResult<(PrimaryKey<'a>, bool), UserError>> {
     // make sure that we have a declared primary key, either fetching it from the index or attempting to guess it.
 
@@ -769,12 +770,18 @@ pub fn retrieve_or_guess_primary_key<'a>(
                 None => return Ok(Err(UserError::NoPrimaryKeyCandidateFound)),
             };
 
-            let mut guesses: Vec<&str> = first_document
+            let guesses: Result<Vec<&str>> = first_document
                 .keys()
-                .map(AsRef::as_ref)
-                .filter(|name| name.to_lowercase().ends_with(DEFAULT_PRIMARY_KEY))
+                .filter_map(|name| {
+                    let Some(_) = new_fields_ids_map.insert(name) else {
+                        return Some(Err(UserError::AttributeLimitReached.into()));
+                    };
+                    name.to_lowercase().ends_with(DEFAULT_PRIMARY_KEY).then_some(Ok(name))
+                })
                 .collect();
 
+            let mut guesses = guesses?;
+
             // sort the keys in lexicographical order, so that fields are always in the same order.
             guesses.sort_unstable();