Index: clang/lib/Basic/SourceManager.cpp
===================================================================
--- clang/lib/Basic/SourceManager.cpp
+++ clang/lib/Basic/SourceManager.cpp
@@ -1941,6 +1941,10 @@
 
   assert(!MacroArgsCache->empty());
   MacroArgsMap::iterator I = MacroArgsCache->upper_bound(Offset);
+
+  if (I == MacroArgsCache->begin())
+    return Loc;
+
   --I;
 
   unsigned MacroArgBeginOffs = I->first;