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 +145,8 @@
       if os.path.isfile(path):
         results.append((symbol_name,
                       pool.apply_async(_ReadSymbolPage, (path, symbol_name))))
+      else:
+        sys.stderr.write("Could not parse page: %s. Page does not exist.\n" % path)                
 
     # Build map from symbol name to a set of headers.
     symbol_headers = collections.defaultdict(set)