diff --git a/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedDistributedTransactionManager.java b/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedDistributedTransactionManager.java
index f5cdb0bcf9..2dc86ebf42 100644
--- a/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedDistributedTransactionManager.java
+++ b/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedDistributedTransactionManager.java
@@ -21,10 +21,12 @@
 import com.scalar.db.exception.transaction.UnknownTransactionStatusException;
 import com.scalar.db.util.ActiveExpiringMap;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiConsumer;
+import javax.annotation.Nonnull;
 import javax.annotation.concurrent.ThreadSafe;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -103,6 +105,11 @@ public DistributedTransaction resume(String txId) throws TransactionNotFoundExce
                     CoreError.TRANSACTION_NOT_FOUND.buildMessage(), txId));
   }
 
+  /**
+   * The methods of this class are synchronized to be thread-safe because the rollback() method may
+   * be called from the expiration handler in a different thread while other methods are being
+   * executed.
+   */
   @VisibleForTesting
   class ActiveTransaction extends DecoratedDistributedTransaction {
 
@@ -124,7 +131,37 @@ public synchronized List<Result> scan(Scan scan) throws CrudException {
 
     @Override
     public synchronized Scanner getScanner(Scan scan) throws CrudException {
-      return super.getScanner(scan);
+      Scanner scanner = super.getScanner(scan);
+      return new Scanner() {
+        @Override
+        public Optional<Result> one() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            return scanner.one();
+          }
+        }
+
+        @Override
+        public List<Result> all() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            return scanner.all();
+          }
+        }
+
+        @Override
+        public void close() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            scanner.close();
+          }
+        }
+
+        @Nonnull
+        @Override
+        public Iterator<Result> iterator() {
+          synchronized (ActiveTransaction.this) {
+            return scanner.iterator();
+          }
+        }
+      };
     }
 
     /** @deprecated As of release 3.13.0. Will be removed in release 5.0.0. */
diff --git a/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedTwoPhaseCommitTransactionManager.java b/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedTwoPhaseCommitTransactionManager.java
index af5d630ef2..820fba1fcc 100644
--- a/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedTwoPhaseCommitTransactionManager.java
+++ b/core/src/main/java/com/scalar/db/common/ActiveTransactionManagedTwoPhaseCommitTransactionManager.java
@@ -23,10 +23,12 @@
 import com.scalar.db.exception.transaction.ValidationException;
 import com.scalar.db.util.ActiveExpiringMap;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiConsumer;
+import javax.annotation.Nonnull;
 import javax.annotation.concurrent.ThreadSafe;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -109,6 +111,11 @@ public TwoPhaseCommitTransaction resume(String txId) throws TransactionNotFoundE
                     CoreError.TRANSACTION_NOT_FOUND.buildMessage(), txId));
   }
 
+  /**
+   * The methods of this class are synchronized to be thread-safe because the rollback() method may
+   * be called from the expiration handler in a different thread while other methods are being
+   * executed.
+   */
   @VisibleForTesting
   class ActiveTransaction extends DecoratedTwoPhaseCommitTransaction {
 
@@ -130,7 +137,37 @@ public synchronized List<Result> scan(Scan scan) throws CrudException {
 
     @Override
     public synchronized Scanner getScanner(Scan scan) throws CrudException {
-      return super.getScanner(scan);
+      Scanner scanner = super.getScanner(scan);
+      return new Scanner() {
+        @Override
+        public Optional<Result> one() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            return scanner.one();
+          }
+        }
+
+        @Override
+        public List<Result> all() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            return scanner.all();
+          }
+        }
+
+        @Override
+        public void close() throws CrudException {
+          synchronized (ActiveTransaction.this) {
+            scanner.close();
+          }
+        }
+
+        @Nonnull
+        @Override
+        public Iterator<Result> iterator() {
+          synchronized (ActiveTransaction.this) {
+            return scanner.iterator();
+          }
+        }
+      };
     }
 
     /** @deprecated As of release 3.13.0. Will be removed in release 5.0.0. */