diff --git a/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java b/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
index 80edfb9bf18..58b46e44509 100644
--- a/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
+++ b/compiler/backend/src/org/jetbrains/kotlin/codegen/ClassFileFactory.java
@@ -52,13 +52,14 @@ import org.jetbrains.org.objectweb.asm.Type;
 import java.io.File;
 import java.nio.charset.StandardCharsets;
 import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
 
 import static org.jetbrains.kotlin.codegen.JvmCodegenUtil.getMappingFileName;
 
 public class ClassFileFactory implements OutputFileCollection {
     private final GenerationState state;
     private final ClassBuilderFactory builderFactory;
-    private final Map<String, OutAndSourceFileList> generators = new LinkedHashMap<>();
+    private final Map<String, OutAndSourceFileList> generators = Collections.synchronizedMap(new LinkedHashMap<>());
 
     private boolean isDone = false;