diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index 158d0ec..e129499 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -540,6 +540,8 @@
 void Preprocessor::replayPreambleConditionalStack() {
   // Restore the conditional stack from the preamble, if there is one.
   if (PreambleConditionalStack.isReplaying()) {
+    assert(CurPPLexer &&
+           "CurPPLexer is null when calling replayPreambleConditionalStack.");
     CurPPLexer->setConditionalLevels(PreambleConditionalStack.getStack());
     PreambleConditionalStack.doneReplaying();
   }