diff --git a/clang/tools/include-mapping/cppreference_parser.py b/clang/tools/include-mapping/cppreference_parser.py
--- a/clang/tools/include-mapping/cppreference_parser.py
+++ b/clang/tools/include-mapping/cppreference_parser.py
@@ -145,6 +145,9 @@
       if os.path.isfile(path):
         results.append((symbol_name,
                       pool.apply_async(_ReadSymbolPage, (path, symbol_name))))
+      else:
+        sys.stderr.write("Discarding information for symbol: %s. Page %s does not exist.\n" 
+          % (symbol_name, path))                
 
     # Build map from symbol name to a set of headers.
     symbol_headers = collections.defaultdict(set)