diff --git a/clang-tools-extra/clangd/unittests/CMakeLists.txt b/clang-tools-extra/clangd/unittests/CMakeLists.txt
--- a/clang-tools-extra/clangd/unittests/CMakeLists.txt
+++ b/clang-tools-extra/clangd/unittests/CMakeLists.txt
@@ -63,6 +63,7 @@
   IndexTests.cpp
   JSONTransportTests.cpp
   LSPClient.cpp
+  ModulesTests.cpp
   ParsedASTTests.cpp
   PathMappingTests.cpp
   PreambleTests.cpp
diff --git a/clang-tools-extra/clangd/unittests/ModulesTests.cpp b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
@@ -0,0 +1,45 @@
+//===-- ModulesTests.cpp  ---------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Annotations.h"
+#include "TestFS.h"
+#include "TestTU.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+#include <memory>
+#include <string>
+
+namespace clang {
+namespace clangd {
+namespace {
+
+TEST(Modules, TextualIncludeInPreamble) {
+  TestTU TU = TestTU::withCode(R"cpp(
+#include "Textual.h"
+
+void foo() {}
+)cpp");
+  TU.ExtraArgs.push_back("-fmodules");
+  TU.ExtraArgs.push_back("-fmodule-name=M");
+  TU.ExtraArgs.push_back("-fmodule-map-file=m.modulemap");
+  TU.AdditionalFiles["Textual.h"] = "void foo();";
+  TU.AdditionalFiles["m.modulemap"] = R"modulemap(
+module M {
+  module Textual {
+    textual header "Textual.h"
+  }
+}
+)modulemap";
+  // Test that we do not crash.
+  TU.index();
+}
+
+} // namespace
+} // namespace clangd
+} // namespace clang
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
@@ -208,6 +208,11 @@
     Callbacks.AfterPCHEmitted(Writer);
   }
 
+  bool BeginSourceFileAction(CompilerInstance &CI) override {
+    CI.getLangOpts().CompilingPCH = true;
+    return ASTFrontendAction::BeginSourceFileAction(CI);
+  }
+
   bool shouldEraseOutputFiles() override { return !hasEmittedPreamblePCH(); }
   bool hasCodeCompletionSupport() const override { return false; }
   bool hasASTFileSupport() const override { return false; }