diff --git a/compiler/integration-tests/src/org/jetbrains/kotlin/KotlinIntegrationTestBase.java b/compiler/integration-tests/src/org/jetbrains/kotlin/KotlinIntegrationTestBase.java
index 11652b1f4cc..9ffb9cc3091 100644
--- a/compiler/integration-tests/src/org/jetbrains/kotlin/KotlinIntegrationTestBase.java
+++ b/compiler/integration-tests/src/org/jetbrains/kotlin/KotlinIntegrationTestBase.java
@@ -156,7 +156,7 @@ public abstract class KotlinIntegrationTestBase {
                 }
             }
 
-            private void appendToContent(StringBuilder content, String prefix, String line) {
+            private synchronized void appendToContent(StringBuilder content, String prefix, String line) {
                 content.append(prefix);
                 content.append(StringUtil.trimTrailing(line));
                 content.append("\n");