diff --git a/java/java-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfo.java b/java/java-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfo.java
index b54a18f42467..1594b3b3674e 100644
--- a/java/java-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfo.java
+++ b/java/java-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfo.java
@@ -58,7 +58,7 @@ class AnchorElementInfo extends SelfElementInfo {
     if (stubId != -1) {
       PsiFile file = SelfElementInfo.restoreFileFromVirtual(getVirtualFile(), myProject);
       if (!(file instanceof PsiFileWithStubSupport)) return null;
-      return PsiAnchor.restoreFromStubIndex((PsiFileWithStubSupport)file, stubId, myStubElementType);
+      return PsiAnchor.restoreFromStubIndex((PsiFileWithStubSupport)file, stubId, myStubElementType, false);
     }
     if (!mySyncMarkerIsValid) return null;
     PsiFile file = SelfElementInfo.restoreFileFromVirtual(getVirtualFile(), myProject);
diff --git a/platform/lang-impl/src/com/intellij/psi/PsiAnchor.java b/platform/lang-impl/src/com/intellij/psi/PsiAnchor.java
index c50727ee7fa2..f104591310b7 100644
--- a/platform/lang-impl/src/com/intellij/psi/PsiAnchor.java
+++ b/platform/lang-impl/src/com/intellij/psi/PsiAnchor.java
@@ -327,8 +327,11 @@ public abstract class PsiAnchor {
   @Nullable
   public static PsiElement restoreFromStubIndex(PsiFileWithStubSupport fileImpl,
                                                 int index,
-                                                @NotNull IStubElementType elementType) {
-    if (fileImpl == null) return null;
+                                                @NotNull IStubElementType elementType, boolean throwIfNull) {
+    if (fileImpl == null) {
+      if (throwIfNull) throw new AssertionError("Null file");
+      return null;
+    }
     StubTree tree = fileImpl.getStubTree();
 
     boolean foreign = tree == null;
@@ -338,26 +341,36 @@ public abstract class PsiAnchor {
         tree = ((PsiFileImpl)fileImpl).calcStubTree();
       }
       else {
+        if (throwIfNull) throw new AssertionError("Not PsiFileImpl: " + fileImpl.getClass());
         return null;
       }
     }
 
     List<StubElement<?>> list = tree.getPlainList();
-    if (index >= list.size()) return null;
+    if (index >= list.size()) {
+      if (throwIfNull) throw new AssertionError("Too large index: " + index + ">=" + list.size());
+      return null;
+    }
     StubElement stub = list.get(index);
 
-    if (stub.getStubType() != elementType) return null;
+    if (stub.getStubType() != elementType) {
+      if (throwIfNull) throw new AssertionError("Element type mismatch: " + stub.getStubType() + "!=" + elementType);
+      return null;
+    }
 
     if (foreign) {
       final PsiElement cachedPsi = ((StubBase)stub).getCachedPsi();
       if (cachedPsi != null) return cachedPsi;
 
       final ASTNode ast = fileImpl.findTreeForStub(tree, stub);
-      return ast != null ? ast.getPsi() : null;
-    }
-    else {
-      return stub.getPsi();
+      if (ast != null) {
+        return ast.getPsi();
+      }
+
+      if (throwIfNull) throw new AssertionError("No AST for stub");
+      return null;
     }
+    return stub.getPsi();
   }
 
   public static class StubIndexReference extends PsiAnchor {
@@ -396,11 +409,26 @@ public abstract class PsiAnchor {
       return ApplicationManager.getApplication().runReadAction(new NullableComputable<PsiElement>() {
         @Override
         public PsiElement compute() {
-          return restoreFromStubIndex((PsiFileWithStubSupport)getFile(), myIndex, myElementType);
+          return restoreFromStubIndex((PsiFileWithStubSupport)getFile(), myIndex, myElementType, false);
         }
       });
     }
 
+    public String diagnoseNull() {
+      try {
+        PsiElement element = ApplicationManager.getApplication().runReadAction(new NullableComputable<PsiElement>() {
+          @Override
+          public PsiElement compute() {
+            return restoreFromStubIndex((PsiFileWithStubSupport)getFile(), myIndex, myElementType, true);
+          }
+        });
+        return "No diagnostics, element=" + element + "@" + (element == null ? 0 : System.identityHashCode(element));
+      }
+      catch (AssertionError e) {
+        return e.getMessage();
+      }
+    }
+
     @Override
     public boolean equals(final Object o) {
       if (this == o) return true;