Index: include/clang/CrossTU/CrossTranslationUnit.h
===================================================================
--- include/clang/CrossTU/CrossTranslationUnit.h
+++ include/clang/CrossTU/CrossTranslationUnit.h
@@ -40,7 +40,6 @@
   multiple_definitions,
   missing_definition,
   failed_import,
-  failed_to_get_external_ast,
   failed_to_generate_usr
 };
 
@@ -118,8 +117,8 @@
   /// \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. The pointer should not be 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
@@ -51,8 +51,6 @@
       return "Missing definition from the index file.";
     case index_error_code::failed_import:
       return "Failed to import the definition.";
-    case index_error_code::failed_to_get_external_ast:
-      return "Failed to load external AST source.";
     case index_error_code::failed_to_generate_usr:
       return "Failed to generate USR.";
     }
@@ -160,9 +158,6 @@
   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());
 
@@ -240,6 +235,7 @@
   } else {
     Unit = FnUnitCacheEntry->second;
   }
+  assert(Unit);
   return Unit;
 }