diff --git a/lib/Lex/MacroArgs.cpp b/lib/Lex/MacroArgs.cpp
index 2646a61..cba69b7 100644
--- a/lib/Lex/MacroArgs.cpp
+++ b/lib/Lex/MacroArgs.cpp
@@ -72,6 +72,7 @@
     if (Result->is(tok::eof))
       --Arg;
   }
+  assert(Result < Start+NumUnexpArgTokens && "Invalid arg #");
   return Result;
 }
 
diff --git a/lib/Lex/PPMacroExpansion.cpp b/lib/Lex/PPMacroExpansion.cpp
index 48c166d..18dcfb2 100644
--- a/lib/Lex/PPMacroExpansion.cpp
+++ b/lib/Lex/PPMacroExpansion.cpp
@@ -427,6 +427,11 @@
     Tok.setLocation(EndLoc);
     Tok.setLength(0);
     ArgTokens.push_back(Tok);
+
+    // If we expect two arguments, add both as empty.
+    if (NumActuals == 0 && MinArgsExpected == 2)
+      ArgTokens.push_back(Tok);
+    
   } else if (NumActuals > MinArgsExpected && !MI->isVariadic()) {
     // Emit the diagnostic at the macro name in case there is a missing ).
     // Emitting it at the , could be far away from the macro name.
diff --git a/test/Preprocessor/macro_expand.c b/test/Preprocessor/macro_expand.c
index c2c76bd..74b3922 100644
--- a/test/Preprocessor/macro_expand.c
+++ b/test/Preprocessor/macro_expand.c
@@ -14,3 +14,6 @@
 B: f(f))
 C: for(for))
 
+// rdar://6880648
+#define f(x,y...) y
+f()