diff --git a/clang-tools-extra/clang-doc/HTMLGenerator.cpp b/clang-tools-extra/clang-doc/HTMLGenerator.cpp
index e110f312d10c..4ab962be7864 100644
--- a/clang-tools-extra/clang-doc/HTMLGenerator.cpp
+++ b/clang-tools-extra/clang-doc/HTMLGenerator.cpp
@@ -899,7 +899,7 @@ static llvm::Error SerializeIndex(ClangDocContext &CDCtx) {
   }
   CDCtx.Idx.sort();
   llvm::json::OStream J(OS, 2);
-  std::function<void(Index)> IndexToJSON = [&](Index I) {
+  std::function<void(Index)> IndexToJSON = [&](const Index &I) {
     J.object([&] {
       J.attribute("USR", toHex(llvm::toStringRef(I.USR)));
       J.attribute("Name", I.Name);