diff --git a/exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/transactions/TransactionApi.kt b/exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/transactions/TransactionApi.kt
index 6374ae0639..6e7dbaf14e 100644
--- a/exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/transactions/TransactionApi.kt
+++ b/exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/transactions/TransactionApi.kt
@@ -58,14 +58,14 @@ interface TransactionManager {
         internal val currentDefaultDatabase = AtomicReference<Database>()
 
         var defaultDatabase: Database?
-            get() = currentDefaultDatabase.get() ?: databases.firstOrNull()
-            set(value) { currentDefaultDatabase.set(value) }
+            @Synchronized get() = currentDefaultDatabase.get() ?: databases.firstOrNull()
+            @Synchronized set(value) { currentDefaultDatabase.set(value) }
 
         private val databases = ConcurrentLinkedDeque<Database>()
 
         private val registeredDatabases = ConcurrentHashMap<Database, TransactionManager>()
 
-        fun registerManager(database: Database, manager: TransactionManager) {
+        @Synchronized fun registerManager(database: Database, manager: TransactionManager) {
             if (defaultDatabase == null) {
                 currentThreadManager.remove()
             }
@@ -73,7 +73,7 @@ interface TransactionManager {
             databases.push(database)
         }
 
-        fun closeAndUnregister(database: Database) {
+        @Synchronized fun closeAndUnregister(database: Database) {
             val manager = registeredDatabases[database]
             manager?.let {
                 registeredDatabases.remove(database)
@@ -87,7 +87,7 @@ interface TransactionManager {
 
         fun managerFor(database: Database?) = if (database != null) registeredDatabases[database] else manager
 
-        internal val currentThreadManager = object : ThreadLocal<TransactionManager>() {
+        private val currentThreadManager = object : ThreadLocal<TransactionManager>() {
             override fun initialValue(): TransactionManager {
                 return defaultDatabase?.let { registeredDatabases.getValue(it) } ?: NotInitializedManager
             }