diff --git a/db/flush_scheduler.cc b/db/flush_scheduler.cc
index 8735a6b369..9c6c04efe3 100644
--- a/db/flush_scheduler.cc
+++ b/db/flush_scheduler.cc
@@ -13,9 +13,11 @@ namespace rocksdb {
 
 void FlushScheduler::ScheduleFlush(ColumnFamilyData* cfd) {
 #ifndef NDEBUG
-  std::lock_guard<std::mutex> lock(checking_mutex_);
-  assert(checking_set_.count(cfd) == 0);
-  checking_set_.insert(cfd);
+  {
+    std::lock_guard<std::mutex> lock(checking_mutex_);
+    assert(checking_set_.count(cfd) == 0);
+    checking_set_.insert(cfd);
+  }
 #endif  // NDEBUG
   cfd->Ref();
 // Suppress false positive clang analyzer warnings.
@@ -32,9 +34,6 @@ void FlushScheduler::ScheduleFlush(ColumnFamilyData* cfd) {
 }
 
 ColumnFamilyData* FlushScheduler::TakeNextColumnFamily() {
-#ifndef NDEBUG
-  std::lock_guard<std::mutex> lock(checking_mutex_);
-#endif  // NDEBUG
   while (true) {
     if (head_.load(std::memory_order_relaxed) == nullptr) {
       return nullptr;
@@ -47,9 +46,12 @@ ColumnFamilyData* FlushScheduler::TakeNextColumnFamily() {
     delete node;
 
 #ifndef NDEBUG
-    auto iter = checking_set_.find(cfd);
-    assert(iter != checking_set_.end());
-    checking_set_.erase(iter);
+    {
+      std::lock_guard<std::mutex> lock(checking_mutex_);
+      auto iter = checking_set_.find(cfd);
+      assert(iter != checking_set_.end());
+      checking_set_.erase(iter);
+    }
 #endif  // NDEBUG
 
     if (!cfd->IsDropped()) {
@@ -65,12 +67,12 @@ ColumnFamilyData* FlushScheduler::TakeNextColumnFamily() {
 }
 
 bool FlushScheduler::Empty() {
-#ifndef NDEBUG
-  std::lock_guard<std::mutex> lock(checking_mutex_);
-#endif  // NDEBUG
   auto rv = head_.load(std::memory_order_relaxed) == nullptr;
 #ifndef NDEBUG
-  assert(rv == checking_set_.empty());
+  std::lock_guard<std::mutex> lock(checking_mutex_);
+  // Empty is allowed to be called concurrnetly with ScheduleFlush. It would
+  // only miss the recent schedules.
+  assert((rv == checking_set_.empty()) || rv);
 #endif  // NDEBUG
   return rv;
 }