diff --git a/milli/src/update/index_documents/mod.rs b/milli/src/update/index_documents/mod.rs
index 58e964986..ed2347b25 100644
--- a/milli/src/update/index_documents/mod.rs
+++ b/milli/src/update/index_documents/mod.rs
@@ -20,7 +20,8 @@ use typed_chunk::{write_typed_chunk_into_index, TypedChunk};
 pub use self::helpers::{
     as_cloneable_grenad, create_sorter, create_writer, fst_stream_into_hashset,
     fst_stream_into_vec, merge_cbo_roaring_bitmaps, merge_roaring_bitmaps,
-    sorter_into_lmdb_database, write_into_lmdb_database, writer_into_reader, ClonableMmap, MergeFn,
+    sorter_into_lmdb_database, valid_lmdb_key, write_into_lmdb_database, writer_into_reader,
+    ClonableMmap, MergeFn,
 };
 use self::helpers::{grenad_obkv_into_chunks, GrenadParameters};
 pub use self::transform::{Transform, TransformOutput};
diff --git a/milli/src/update/word_prefix_docids.rs b/milli/src/update/word_prefix_docids.rs
index 2887b5583..1002c13cf 100644
--- a/milli/src/update/word_prefix_docids.rs
+++ b/milli/src/update/word_prefix_docids.rs
@@ -5,7 +5,8 @@ use heed::types::{ByteSlice, Str};
 use heed::Database;
 
 use crate::update::index_documents::{
-    create_sorter, merge_roaring_bitmaps, sorter_into_lmdb_database, CursorClonableMmap, MergeFn,
+    create_sorter, merge_roaring_bitmaps, sorter_into_lmdb_database, valid_lmdb_key,
+    CursorClonableMmap, MergeFn,
 };
 use crate::{Result, RoaringBitmapCodec};
 
@@ -124,7 +125,9 @@ fn write_prefixes_in_sorter(
 ) -> Result<()> {
     for (key, data_slices) in prefixes.drain() {
         for data in data_slices {
-            sorter.insert(&key, data)?;
+            if valid_lmdb_key(&key) {
+                sorter.insert(&key, data)?;
+            }
         }
     }
 
diff --git a/milli/src/update/word_prefix_pair_proximity_docids.rs b/milli/src/update/word_prefix_pair_proximity_docids.rs
index be0ddf005..72b41c472 100644
--- a/milli/src/update/word_prefix_pair_proximity_docids.rs
+++ b/milli/src/update/word_prefix_pair_proximity_docids.rs
@@ -7,8 +7,8 @@ use log::debug;
 use slice_group_by::GroupBy;
 
 use crate::update::index_documents::{
-    create_sorter, merge_cbo_roaring_bitmaps, sorter_into_lmdb_database, CursorClonableMmap,
-    MergeFn,
+    create_sorter, merge_cbo_roaring_bitmaps, sorter_into_lmdb_database, valid_lmdb_key,
+    CursorClonableMmap, MergeFn,
 };
 use crate::{Index, Result, StrStrU8Codec};
 
@@ -188,7 +188,9 @@ fn write_prefixes_in_sorter(
 ) -> Result<()> {
     for (key, data_slices) in prefixes.drain() {
         for data in data_slices {
-            sorter.insert(&key, data)?;
+            if valid_lmdb_key(&key) {
+                sorter.insert(&key, data)?;
+            }
         }
     }
 
diff --git a/milli/src/update/words_prefix_position_docids.rs b/milli/src/update/words_prefix_position_docids.rs
index 77e9e7c29..b2b24084d 100644
--- a/milli/src/update/words_prefix_position_docids.rs
+++ b/milli/src/update/words_prefix_position_docids.rs
@@ -11,8 +11,8 @@ use crate::error::SerializationError;
 use crate::heed_codec::StrBEU32Codec;
 use crate::index::main_key::WORDS_PREFIXES_FST_KEY;
 use crate::update::index_documents::{
-    create_sorter, merge_cbo_roaring_bitmaps, sorter_into_lmdb_database, CursorClonableMmap,
-    MergeFn,
+    create_sorter, merge_cbo_roaring_bitmaps, sorter_into_lmdb_database, valid_lmdb_key,
+    CursorClonableMmap, MergeFn,
 };
 use crate::{Index, Result};
 
@@ -167,7 +167,9 @@ fn write_prefixes_in_sorter(
 ) -> Result<()> {
     for (key, data_slices) in prefixes.drain() {
         for data in data_slices {
-            sorter.insert(&key, data)?;
+            if valid_lmdb_key(&key) {
+                sorter.insert(&key, data)?;
+            }
         }
     }