Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -492,6 +492,7 @@
 
   ++NumSkipped;
   assert(!CurTokenLexer && CurPPLexer && "Lexing a macro, not a file?");
+  assert(CurLexer && "Invalid lexer value");
 
   if (PreambleConditionalStack.reachedEOFWhileSkipping())
     PreambleConditionalStack.clearSkipInfo();