diff --git a/clang/include/clang/Frontend/PrecompiledPreamble.h b/clang/include/clang/Frontend/PrecompiledPreamble.h
--- a/clang/include/clang/Frontend/PrecompiledPreamble.h
+++ b/clang/include/clang/Frontend/PrecompiledPreamble.h
@@ -274,7 +274,7 @@
 public:
   virtual ~PreambleCallbacks() = default;
 
-  /// Called before FrontendAction::BeginSourceFile.
+  /// Called before FrontendAction::Execute.
   /// Can be used to store references to various CompilerInstance fields
   /// (e.g. SourceManager) that may be interesting to the consumers of other
   /// callbacks.
@@ -291,7 +291,7 @@
   /// used instead, but having only this method allows a simpler API.
   virtual void HandleTopLevelDecl(DeclGroupRef DG);
   /// Creates wrapper class for PPCallbacks so we can also process information
-  /// about includes that are inside of a preamble
+  /// about includes that are inside of a preamble. Called after BeforeExecute.
   virtual std::unique_ptr<PPCallbacks> createPPCallbacks();
   /// The returned CommentHandler will be added to the preprocessor if not null.
   virtual CommentHandler *getCommentHandler();
diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -412,10 +412,13 @@
   std::unique_ptr<PrecompilePreambleAction> Act;
   Act.reset(new PrecompilePreambleAction(
       StoreInMemory ? &Storage.asMemory().Data : nullptr, Callbacks));
-  Callbacks.BeforeExecute(*Clang);
   if (!Act->BeginSourceFile(*Clang.get(), Clang->getFrontendOpts().Inputs[0]))
     return BuildPreambleError::BeginSourceFileFailed;
 
+  // Performed after BeginSourceFile to ensure Clang->Preprocessor can be
+  // referenced in the callback.
+  Callbacks.BeforeExecute(*Clang);
+
   std::unique_ptr<PPCallbacks> DelegatedPPCallbacks =
       Callbacks.createPPCallbacks();
   if (DelegatedPPCallbacks)