diff --git a/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndex.java b/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndex.java
index f41adec1079f..de6c035984c1 100644
--- a/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndex.java
+++ b/platform/lang-impl/src/com/intellij/util/indexing/FileBasedIndex.java
@@ -442,8 +442,10 @@ public class FileBasedIndex implements ApplicationComponent {
   private <K, V> UpdatableIndex<K, V, FileContent> createIndex(final ID<K, V> indexId, final FileBasedIndexExtension<K, V> extension, final MemoryIndexStorage<K, V> storage) throws IOException {
     final MapReduceIndex<K, V, FileContent> index;
     if (extension instanceof CustomImplementationFileBasedIndexExtension) {
-      final UpdatableIndex<K, V, FileContent> custom =
-        ((CustomImplementationFileBasedIndexExtension<K, V, FileContent>)extension).createIndexImplementation(indexId, this, storage);
+      final UpdatableIndex<K, V, FileContent> custom = ((CustomImplementationFileBasedIndexExtension<K, V, FileContent>)extension).createIndexImplementation(indexId, this, storage);
+      
+      assert custom != null : "Custom index implementation must not be null; index: " + indexId;
+      
       if (!(custom instanceof MapReduceIndex)) {
         return custom;
       }