diff --git a/tools/index-test/index-test.cpp b/tools/index-test/index-test.cpp
index 63d37b2..5606e75 100644
--- a/tools/index-test/index-test.cpp
+++ b/tools/index-test/index-test.cpp
@@ -88,6 +88,18 @@
   if (!PointAtLocation.empty()) {
     const std::string &Filename = PointAtLocation[0].FileName;
     const FileEntry *File = FileMgr.getFile(Filename);
+
+    // Safety check. Using an out-of-date AST file will only lead to crashes
+    // or incorrect results.
+    // FIXME: Check all the source files that make up the AST file.
+    const FileEntry *ASTFile = FileMgr.getFile(InFile);
+    if (File->getModificationTime() > ASTFile->getModificationTime()) {
+      llvm::errs() << "[" << InFile << "] Error: " <<
+        "Pointing at a source file which was modified after creating "
+        "the AST file\n";
+      return 1;
+    }
+
     if (File == 0) {
       llvm::errs() << "File '" << Filename << "' does not exist\n";
       return 1;