diff --git a/clang/lib/Lex/PPCaching.cpp b/clang/lib/Lex/PPCaching.cpp
index f5e8cdc..9758557 100644
--- a/clang/lib/Lex/PPCaching.cpp
+++ b/clang/lib/Lex/PPCaching.cpp
@@ -105,8 +105,10 @@
 }
 
 void Preprocessor::EnterCachingLexMode() {
-  if (InCachingLexMode())
+  if (InCachingLexMode()) {
+    assert(CurLexerKind == CLK_CachingLexer && "Unexpected lexer kind");
     return;
+  }
 
   PushIncludeMacroStack();
   CurLexerKind = CLK_CachingLexer;