Index: cfe/trunk/lib/Tooling/InterpolatingCompilationDatabase.cpp
===================================================================
--- cfe/trunk/lib/Tooling/InterpolatingCompilationDatabase.cpp
+++ cfe/trunk/lib/Tooling/InterpolatingCompilationDatabase.cpp
@@ -150,7 +150,8 @@
     // spelling of each argument; re-rendering is lossy for aliased flags.
     // E.g. in CL mode, /W4 maps to -Wall.
     auto OptTable = clang::driver::createDriverOptTable();
-    Cmd.CommandLine.emplace_back(OldArgs.front());
+    if (!OldArgs.empty())
+      Cmd.CommandLine.emplace_back(OldArgs.front());
     for (unsigned Pos = 1; Pos < OldArgs.size();) {
       using namespace driver::options;
 
@@ -243,7 +244,8 @@
     }
 
     // Otherwise just check the clang executable file name.
-    return llvm::sys::path::stem(CmdLine.front()).endswith_lower("cl");
+    return !CmdLine.empty() &&
+           llvm::sys::path::stem(CmdLine.front()).endswith_lower("cl");
   }
 
   // Map the language from the --std flag to that of the -x flag.
Index: cfe/trunk/lib/Tooling/Tooling.cpp
===================================================================
--- cfe/trunk/lib/Tooling/Tooling.cpp
+++ cfe/trunk/lib/Tooling/Tooling.cpp
@@ -481,7 +481,7 @@
       if (OverlayFileSystem->setCurrentWorkingDirectory(
               CompileCommand.Directory))
         llvm::report_fatal_error("Cannot chdir into \"" +
-                                 Twine(CompileCommand.Directory) + "\n!");
+                                 Twine(CompileCommand.Directory) + "\"!");
 
       // Now fill the in-memory VFS with the relative file mappings so it will
       // have the correct relative paths. We never remove mappings but that
Index: clang-tools-extra/trunk/test/clang-tidy/empty-database.cpp
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/empty-database.cpp
+++ clang-tools-extra/trunk/test/clang-tidy/empty-database.cpp
@@ -0,0 +1,3 @@
+// RUN: not clang-tidy -p %S/empty-database %s 2>&1 | FileCheck %s
+
+// CHECK: LLVM ERROR: Cannot chdir into ""!
Index: clang-tools-extra/trunk/test/clang-tidy/empty-database/compile_commands.json
===================================================================
--- clang-tools-extra/trunk/test/clang-tidy/empty-database/compile_commands.json
+++ clang-tools-extra/trunk/test/clang-tidy/empty-database/compile_commands.json
@@ -0,0 +1,4 @@
+[{
+    "directory":"",
+    "file":"/tmp/","arguments":[]
+}]