Index: clangd/ClangdLSPServer.h
===================================================================
--- clangd/ClangdLSPServer.h
+++ clangd/ClangdLSPServer.h
@@ -126,9 +126,10 @@
     void setExtraFlagsForFile(PathRef File,
                               std::vector<std::string> ExtraFlags);
 
-    /// Set the compile commands directory to \p P.
+    /// Set the compile commands directory to \p P.  An empty Optional value
+    /// means to not use an explicit compile commands directory path.
     /// Only valid for directory-based CDB, no-op and error log on InMemoryCDB;
-    void setCompileCommandsDir(Path P);
+    void setCompileCommandsDir(llvm::Optional<Path> P);
 
     /// Returns a CDB that should be used to get compile commands for the
     /// current instance of ClangdLSPServer.
Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -613,14 +613,14 @@
   CachingCDB->invalidate(File);
 }
 
-void ClangdLSPServer::CompilationDB::setCompileCommandsDir(Path P) {
+void ClangdLSPServer::CompilationDB::setCompileCommandsDir(llvm::Optional<Path> P) {
   if (!IsDirectoryBased) {
     elog("Trying to set compile commands dir while using in-memory compilation "
          "database");
     return;
   }
   static_cast<DirectoryBasedGlobalCompilationDatabase *>(CDB.get())
-      ->setCompileCommandsDir(P);
+      ->setCompileCommandsDir(std::move (P));
   CachingCDB->clear();
 }
 
Index: clangd/GlobalCompilationDatabase.h
===================================================================
--- clangd/GlobalCompilationDatabase.h
+++ clangd/GlobalCompilationDatabase.h
@@ -63,7 +63,7 @@
   tooling::CompileCommand getFallbackCommand(PathRef File) const override;
 
   /// Set the compile commands directory to \p P.
-  void setCompileCommandsDir(Path P);
+  void setCompileCommandsDir(llvm::Optional<Path> P);
 
   /// Sets the extra flags that should be added to a file.
   void setExtraFlagsForFile(PathRef File, std::vector<std::string> ExtraFlags);
Index: clangd/GlobalCompilationDatabase.cpp
===================================================================
--- clangd/GlobalCompilationDatabase.cpp
+++ clangd/GlobalCompilationDatabase.cpp
@@ -60,7 +60,7 @@
   return C;
 }
 
-void DirectoryBasedGlobalCompilationDatabase::setCompileCommandsDir(Path P) {
+void DirectoryBasedGlobalCompilationDatabase::setCompileCommandsDir(llvm::Optional<Path> P) {
   std::lock_guard<std::mutex> Lock(Mutex);
   CompileCommandsDir = P;
   CompilationDatabases.clear();
Index: clangd/Protocol.h
===================================================================
--- clangd/Protocol.h
+++ clangd/Protocol.h
@@ -358,7 +358,14 @@
 /// "initialize" request and for the "workspace/didChangeConfiguration"
 /// notification since the data received is described as 'any' type in LSP.
 struct ClangdConfigurationParamsChange {
-  llvm::Optional<std::string> compilationDatabasePath;
+  /// Path to the directory containing the compile_commands.json file to use.
+  /// The outer Optional is empty if the compilationDatabasePath field was not
+  /// present at all in the request.
+  /// The inner Optional is empty if the compilationDatabasePath field was
+  /// preset, but the value was null.
+  /// If the value of the compilationDatabasePath in the request is a string,
+  /// both Optionals are instantiated.
+  llvm::Optional<llvm::Optional<std::string>> compilationDatabasePath;
 
   // The changes that happened to the compilation database.
   // The key of the map is a file name.
Index: clangd/Protocol.cpp
===================================================================
--- clangd/Protocol.cpp
+++ clangd/Protocol.cpp
@@ -609,11 +609,22 @@
          O.map("compilationCommand", CDbUpdate.compilationCommand);
 }
 
-bool fromJSON(const json::Value &Params,
+bool fromJSON(const json::Value &Settings,
               ClangdConfigurationParamsChange &CCPC) {
-  json::ObjectMapper O(Params);
-  return O && O.map("compilationDatabasePath", CCPC.compilationDatabasePath) &&
-         O.map("compilationDatabaseChanges", CCPC.compilationDatabaseChanges);
+  json::ObjectMapper O(Settings);
+  const json::Value *P = Settings.getAsObject()->get("compilationDatabasePath");
+
+  if (P) {
+    // The field is present.
+    CCPC.compilationDatabasePath.emplace();
+    llvm::Optional<StringRef> S = P->getAsString();
+    if (S) {
+      // The field has a string value.
+      CCPC.compilationDatabasePath->emplace(*S);
+    }
+  }
+
+  return O && O.map("compilationDatabaseChanges", CCPC.compilationDatabaseChanges);
 }
 
 bool fromJSON(const json::Value &Params, ReferenceParams &R) {
Index: test/clangd/compile-commands-path.test
===================================================================
--- test/clangd/compile-commands-path.test
+++ test/clangd/compile-commands-path.test
@@ -39,4 +39,11 @@
 # CHECK-NEXT:       {
 # CHECK-NEXT:         "message": "MACRO is two",
 ---
+{"jsonrpc":"2.0","id":0,"method":"workspace/didChangeConfiguration","params":{"settings":{"compilationDatabasePath":null}}}
+# CHECK:   "method": "textDocument/publishDiagnostics",
+# CHECK-NEXT:   "params": {
+# CHECK-NEXT:     "diagnostics": [
+# CHECK-NEXT:       {
+# CHECK-NEXT:         "message": "MACRO is not defined",
+---
 {"jsonrpc":"2.0","id":10000,"method":"shutdown"}