diff --git a/lib/Lex/PPLexerChange.cpp b/lib/Lex/PPLexerChange.cpp
index d4defdd71a7b8c381e20b669527d77d934eb5a79..a27bcd000a406e071211e86b68b8509c82c11eaf 100644
--- a/lib/Lex/PPLexerChange.cpp
+++ b/lib/Lex/PPLexerChange.cpp
@@ -685,8 +685,11 @@ void Preprocessor::LeaveSubmodule() {
         // FIXME: Issue a warning if multiple headers for the same submodule
         // define a macro, rather than silently ignoring all but the first.
         bool IsNew;
-        addModuleMacro(Info.M, II, Def, Macro.second.getOverriddenMacros(),
-                       IsNew);
+        // Don't bother creating a module macro if it would represent a #undef
+        // that doesn't override anything.
+        if (Def || !Macro.second.getOverriddenMacros().empty())
+          addModuleMacro(Info.M, II, Def, Macro.second.getOverriddenMacros(),
+                         IsNew);
         break;
       }
     }