diff --git a/src/transaction/buffer.rs b/src/transaction/buffer.rs
index 1d7f61b..7f47790 100644
--- a/src/transaction/buffer.rs
+++ b/src/transaction/buffer.rs
@@ -17,7 +17,7 @@ struct InnerBuffer {
 impl InnerBuffer {
     fn insert(&mut self, key: impl Into<Key>, entry: BufferEntry) {
         let key = key.into();
-        if !matches!(entry, BufferEntry::Cached(_)) {
+        if !matches!(entry, BufferEntry::Cached(_) | BufferEntry::CheckNotExist) {
             self.primary_key.get_or_insert_with(|| key.clone());
         }
         self.entry_map.insert(key, entry);
@@ -31,17 +31,17 @@ impl InnerBuffer {
 /// A caching layer which buffers reads and writes in a transaction.
 #[derive(Default)]
 pub struct Buffer {
-    mutations: Mutex<InnerBuffer>,
+    inner: Mutex<InnerBuffer>,
 }
 
 impl Buffer {
     /// Get the primary key of the buffer.
     pub async fn get_primary_key(&self) -> Option<Key> {
-        self.mutations.lock().await.primary_key.clone()
+        self.inner.lock().await.primary_key.clone()
     }
     /// Get the primary key of the buffer, if not exists, use `key` as the primary key.
     pub async fn get_primary_key_or(&self, key: &Key) -> Key {
-        self.mutations.lock().await.get_primary_key_or(key).clone()
+        self.inner.lock().await.get_primary_key_or(key).clone()
     }
 
     /// Get a value from the buffer.
@@ -64,7 +64,7 @@ impl Buffer {
             MutationValue::Determined(value) => Ok(value),
             MutationValue::Undetermined => {
                 let value = f(key.clone()).await?;
-                let mut mutations = self.mutations.lock().await;
+                let mut mutations = self.inner.lock().await;
                 Self::update_cache(&mut mutations, key, value.clone());
                 Ok(value)
             }
@@ -85,7 +85,7 @@ impl Buffer {
         Fut: Future<Output = Result<Vec<KvPair>>>,
     {
         let (cached_results, undetermined_keys) = {
-            let mutations = self.mutations.lock().await;
+            let mutations = self.inner.lock().await;
             // Partition the keys into those we have buffered and those we have to
             // get from the store.
             let (undetermined_keys, cached_results): (
@@ -111,7 +111,7 @@ impl Buffer {
         };
 
         let fetched_results = f(Box::new(undetermined_keys)).await?;
-        let mut mutations = self.mutations.lock().await;
+        let mut mutations = self.inner.lock().await;
         for kvpair in &fetched_results {
             let key = kvpair.0.clone();
             let value = Some(kvpair.1.clone());
@@ -134,7 +134,7 @@ impl Buffer {
         Fut: Future<Output = Result<Vec<KvPair>>>,
     {
         // read from local buffer
-        let mut mutations = self.mutations.lock().await;
+        let mut mutations = self.inner.lock().await;
         let mutation_range = mutations.entry_map.range(range.clone());
 
         // fetch from TiKV
@@ -180,8 +180,8 @@ impl Buffer {
 
     /// Lock the given key if necessary.
     pub async fn lock(&self, key: Key) {
-        let mutations = &mut self.mutations.lock().await;
-        mutations.primary_key.get_or_insert(key.clone());
+        let mutations = &mut self.inner.lock().await;
+        mutations.primary_key.get_or_insert_with(|| key.clone());
         let value = mutations
             .entry_map
             .entry(key)
@@ -195,15 +195,12 @@ impl Buffer {
 
     /// Insert a value into the buffer (does not write through).
     pub async fn put(&self, key: Key, value: Value) {
-        self.mutations
-            .lock()
-            .await
-            .insert(key, BufferEntry::Put(value));
+        self.inner.lock().await.insert(key, BufferEntry::Put(value));
     }
 
     /// Mark a value as Insert mutation into the buffer (does not write through).
     pub async fn insert(&self, key: Key, value: Value) {
-        self.mutations
+        self.inner
             .lock()
             .await
             .insert(key, BufferEntry::Insert(value));
@@ -211,7 +208,7 @@ impl Buffer {
 
     /// Mark a value as deleted.
     pub async fn delete(&self, key: Key) {
-        let mut mutations = self.mutations.lock().await;
+        let mut mutations = self.inner.lock().await;
         let value = mutations
             .entry_map
             .entry(key.clone())
@@ -229,7 +226,7 @@ impl Buffer {
 
     /// Converts the buffered mutations to the proto buffer version
     pub async fn to_proto_mutations(&self) -> Vec<kvrpcpb::Mutation> {
-        self.mutations
+        self.inner
             .lock()
             .await
             .entry_map
@@ -239,7 +236,7 @@ impl Buffer {
     }
 
     async fn get_from_mutations(&self, key: &Key) -> MutationValue {
-        self.mutations
+        self.inner
             .lock()
             .await
             .entry_map
diff --git a/src/transaction/transaction.rs b/src/transaction/transaction.rs
index 1fe8be4..dd5e432 100644
--- a/src/transaction/transaction.rs
+++ b/src/transaction/transaction.rs
@@ -644,7 +644,13 @@ impl<PdC: PdClient> Transaction<PdC> {
         }
 
         let first_key = keys[0].clone();
-        let primary_lock = self.buffer.get_primary_key_or(&first_key).await;
+        // we do not set the primary key here, because pessimistic lock request
+        // can fail, in which case the keys may not be part of the transaction.
+        let primary_lock = self
+            .buffer
+            .get_primary_key()
+            .await
+            .unwrap_or_else(|| first_key.clone());
         let for_update_ts = self.rpc.clone().get_timestamp().await?;
         self.options.push_for_update_ts(for_update_ts.clone());
         let request = new_pessimistic_lock_request(
@@ -664,6 +670,9 @@ impl<PdC: PdClient> Transaction<PdC> {
             .plan();
         let pairs = plan.execute().await;
 
+        // primary key will be set here if needed
+        self.buffer.get_primary_key_or(&first_key).await;
+
         self.start_auto_heartbeat().await;
 
         for key in keys {