diff --git a/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java b/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
index 58b46e44509..68faeff9af6 100644
--- a/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
+++ b/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
@@ -347,7 +347,9 @@ public class ClassFileFactory implements OutputFileCollection {
 
         @Override
         public byte[] asBytes(ClassBuilderFactory factory) {
-            return factory.asBytes(classBuilder);
+            synchronized(this) {
+                return factory.asBytes(classBuilder);
+            }
         }
 
         @Override