diff --git a/java/java-tests/testSrc/com/intellij/index/ConcurrentIndexTest.groovy b/java/java-tests/testSrc/com/intellij/index/ConcurrentIndexTest.groovy
index eb351137a346..126781a1ba23 100644
--- a/java/java-tests/testSrc/com/intellij/index/ConcurrentIndexTest.groovy
+++ b/java/java-tests/testSrc/com/intellij/index/ConcurrentIndexTest.groovy
@@ -14,21 +14,27 @@
  * limitations under the License.
  */
 package com.intellij.index
+
+import com.intellij.lang.FCTSBackedLighterAST
 import com.intellij.openapi.application.ApplicationManager
+import com.intellij.openapi.application.ReadAction
 import com.intellij.openapi.command.WriteCommandAction
 import com.intellij.psi.JavaPsiFacade
 import com.intellij.psi.PsiDocumentManager
 import com.intellij.psi.PsiFile
 import com.intellij.psi.PsiJavaFile
 import com.intellij.psi.impl.PsiDocumentManagerBase
+import com.intellij.psi.impl.search.JavaNullMethodArgumentUtil
 import com.intellij.psi.impl.source.PsiFileImpl
 import com.intellij.psi.search.GlobalSearchScope
 import com.intellij.testFramework.BombedProgressIndicator
 import com.intellij.testFramework.PlatformTestUtil
 import com.intellij.testFramework.SkipSlowTestLocally
 import com.intellij.testFramework.fixtures.JavaCodeInsightFixtureTestCase
+import com.intellij.util.GCUtil
 
 import java.util.concurrent.CountDownLatch
+import java.util.concurrent.Future
 /**
  * @author peter
  */
@@ -170,4 +176,36 @@ class ConcurrentIndexTest extends JavaCodeInsightFixtureTestCase {
       for(future in futuresToWait) future.get()
     }
   }
+
+  void "test concurrent light AST access during uncommitted document indexing"() {
+    def clazz = myFixture.addClass('class Bar { void foo(Object o) {}}')
+
+    def text = " foo(null);";
+    for (i in 0..20) {
+      text = "new Runnable() { void run() {\n " + text + "\n}}.run();"
+    }
+    text = "class Foo {{ " + text * 200 + "}}"
+
+    def file = myFixture.addFileToProject('a.java', text)
+    def document = file.viewProvider.document
+    for (i in 1..5) {
+      WriteCommandAction.runWriteCommandAction project, {
+        document.insertString(document.text.indexOf('null') + 1, ' ')
+        document.insertString(document.text.indexOf('(null') + 1, ' ')
+        PsiDocumentManager.getInstance(project).commitAllDocuments()
+      }
+      GCUtil.tryGcSoftlyReachableObjects()
+
+      assert file.node.lighterAST instanceof FCTSBackedLighterAST
+      List<Future> futures = []
+      futures << ApplicationManager.application.executeOnPooledThread { ReadAction.run {
+        assert !JavaNullMethodArgumentUtil.hasNullArgument(clazz.methods[0], 0)
+      } }
+      futures << ApplicationManager.application.executeOnPooledThread { ReadAction.run {
+        assert JavaPsiFacade.getInstance(project).findClass('Foo', GlobalSearchScope.allScope(project))
+      } }
+      futures.each { it.get() }
+    }
+  }
+
 }
diff --git a/platform/core-impl/src/com/intellij/util/indexing/FileContentImpl.java b/platform/core-impl/src/com/intellij/util/indexing/FileContentImpl.java
index 69d75c85aad2..c3fd6fe7fb5c 100644
--- a/platform/core-impl/src/com/intellij/util/indexing/FileContentImpl.java
+++ b/platform/core-impl/src/com/intellij/util/indexing/FileContentImpl.java
@@ -15,6 +15,7 @@
  */
 package com.intellij.util.indexing;
 
+import com.intellij.lang.FileASTNode;
 import com.intellij.lang.Language;
 import com.intellij.lang.LighterAST;
 import com.intellij.openapi.editor.Document;
@@ -85,12 +86,26 @@ public class FileContentImpl extends UserDataHolderBase implements FileContent {
     LighterAST lighterAST = getUserData(IndexingDataKeys.LIGHTER_AST_NODE_KEY);
     if (lighterAST == null) {
       lighterAST = getPsiFileForPsiDependentIndex().getNode().getLighterAST();
-      assert lighterAST != null;
       putUserData(IndexingDataKeys.LIGHTER_AST_NODE_KEY, lighterAST);
     }
     return lighterAST;
   }
 
+  /**
+   * Expand the AST to ensure {@link com.intellij.lang.FCTSBackedLighterAST} won't be used, because it's not thread-safe,
+   * but unsaved documents may be indexed in many concurrent threads
+   */
+  void ensureThreadSafeLighterAST() {
+    ensureParsed(getPsiFile().getNode());
+    ensureParsed(getPsiFileForPsiDependentIndex().getNode());
+  }
+
+  private static void ensureParsed(@Nullable FileASTNode node) {
+    if (node != null) {
+      node.getFirstChildNode();
+    }
+  }
+
   public PsiFile createFileFromText(@NotNull CharSequence text) {
     Project project = getProject();
     if (project == null) {
@@ -232,6 +247,7 @@ public class FileContentImpl extends UserDataHolderBase implements FileContent {
     myHash = hash;
   }
 
+  @NotNull
   public PsiFile getPsiFileForPsiDependentIndex() {
     Document document = FileDocumentManager.getInstance().getCachedDocument(getFile());
     PsiFile psi = null;
diff --git a/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndexImpl.java b/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndexImpl.java
index 34c01d0ca5d3..df5b30c5d3ef 100644
--- a/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndexImpl.java
+++ b/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndexImpl.java
@@ -1346,6 +1346,7 @@ public class FileBasedIndexImpl extends FileBasedIndex {
         }
 
         initFileContent(newFc, project, dominantContentFile);
+        newFc.ensureThreadSafeLighterAST();
 
         if (content instanceof AuthenticContent) {
           newFc.putUserData(PlatformIdTableBuilding.EDITOR_HIGHLIGHTER,