Index: include/clang/CrossTU/CrossTranslationUnit.h
===================================================================
--- include/clang/CrossTU/CrossTranslationUnit.h
+++ include/clang/CrossTU/CrossTranslationUnit.h
@@ -130,8 +130,9 @@
   /// \p IndexName. In case the declaration is found in the index the
   /// corresponding AST file will be loaded.
   ///
-  /// \return Returns an ASTUnit that contains the definition of the looked up
-  /// function.
+  /// \return Returns a pointer to the ASTUnit that contains the definition of
+  /// the looked up function or an Error.
+  /// The returned pointer is never a nullptr.
   ///
   /// Note that the AST files should also be in the \p CrossTUDir.
   llvm::Expected<ASTUnit *> loadExternalAST(StringRef LookupName,
Index: lib/CrossTU/CrossTranslationUnit.cpp
===================================================================
--- lib/CrossTU/CrossTranslationUnit.cpp
+++ lib/CrossTU/CrossTranslationUnit.cpp
@@ -208,9 +208,6 @@
   if (!ASTUnitOrError)
     return ASTUnitOrError.takeError();
   ASTUnit *Unit = *ASTUnitOrError;
-  if (!Unit)
-    return llvm::make_error<IndexError>(
-        index_error_code::failed_to_get_external_ast);
   assert(&Unit->getFileManager() ==
          &Unit->getASTContext().getSourceManager().getFileManager());
 
@@ -324,6 +321,9 @@
   } else {
     Unit = FnUnitCacheEntry->second;
   }
+  if (!Unit)
+    return llvm::make_error<IndexError>(
+        index_error_code::failed_to_get_external_ast);
   return Unit;
 }