diff --git a/milli/src/update/index_documents/mod.rs b/milli/src/update/index_documents/mod.rs
index 4fbb75d5f..cdea37d54 100644
--- a/milli/src/update/index_documents/mod.rs
+++ b/milli/src/update/index_documents/mod.rs
@@ -353,6 +353,9 @@ where
             total_databases: TOTAL_POSTING_DATABASE_COUNT,
         });
 
+        let previous_words_prefixes_fst =
+            self.index.words_prefixes_fst(self.wtxn)?.map_data(|cow| cow.into_owned())?;
+
         // Run the words prefixes update operation.
         let mut builder = WordsPrefixesFst::new(self.wtxn, self.index);
         if let Some(value) = self.config.words_prefix_threshold {
@@ -389,7 +392,7 @@ where
         builder.chunk_compression_level = self.indexer_config.chunk_compression_level;
         builder.max_nb_chunks = self.indexer_config.max_nb_chunks;
         builder.max_memory = self.indexer_config.max_memory;
-        builder.execute()?;
+        builder.execute(&previous_words_prefixes_fst)?;
 
         databases_seen += 1;
         (self.progress)(UpdateIndexingStep::MergeDataIntoFinalDatabase {
diff --git a/milli/src/update/word_prefix_pair_proximity_docids.rs b/milli/src/update/word_prefix_pair_proximity_docids.rs
index 2dc00fb90..2788d5d35 100644
--- a/milli/src/update/word_prefix_pair_proximity_docids.rs
+++ b/milli/src/update/word_prefix_pair_proximity_docids.rs
@@ -61,7 +61,7 @@ impl<'t, 'u, 'i> WordPrefixPairProximityDocids<'t, 'u, 'i> {
     }
 
     #[logging_timer::time("WordPrefixPairProximityDocids::{}")]
-    pub fn execute(self) -> Result<()> {
+    pub fn execute<A: AsRef<[u8]>>(self, old_prefix_fst: &fst::Set<A>) -> Result<()> {
         debug!("Computing and writing the word prefix pair proximity docids into LMDB on disk...");
 
         self.index.word_prefix_pair_proximity_docids.clear(self.wtxn)?;