diff --git a/dali/operators/reader/loader/discover_files_s3.cc b/dali/operators/reader/loader/discover_files_s3.cc
index 314beba273e..a22f4991327 100644
--- a/dali/operators/reader/loader/discover_files_s3.cc
+++ b/dali/operators/reader/loader/discover_files_s3.cc
@@ -54,8 +54,8 @@ std::vector<FileLabelEntry> s3_discover_files(const std::string &file_root,
         assert(path_elems >= 2);
         if (path_elems > 2)
           return;  // we only look at one subdir level
-        auto subdir = *(p.begin());
-        auto fname = *(++p.begin());
+        const auto& subdir = p.begin()->native();
+        const auto& fname = (++p.begin())->native();
         bool subdir_ok = opts.dir_filters.empty();
         bool fname_ok = opts.file_filters.empty();
         for (auto &filter : opts.dir_filters) {
diff --git a/dali/util/file.cc b/dali/util/file.cc
index e9a6ccb3891..1cc729daddb 100644
--- a/dali/util/file.cc
+++ b/dali/util/file.cc
@@ -32,6 +32,10 @@ std::unique_ptr<FileStream> FileStream::Open(const std::string& uri, FileStream:
   bool is_s3 = uri.rfind("s3://", 0) == 0;
   if (is_s3) {
 #if AWSSDK_ENABLED
+    if (opts.use_mmap)
+      throw std::invalid_argument("use_mmap not supported");
+    if (opts.use_odirect)
+      throw std::invalid_argument("use_odirect not supported");
     return std::make_unique<S3FileStream>(S3ClientManager::Instance().client(), uri, size);
 #else
     throw std::runtime_error("This version of DALI was not built with AWS S3 storage support.");
diff --git a/dali/util/s3_client_manager.h b/dali/util/s3_client_manager.h
index 210f6ee4cb1..4b931883b93 100644
--- a/dali/util/s3_client_manager.h
+++ b/dali/util/s3_client_manager.h
@@ -45,10 +45,9 @@ struct S3ClientManager {
 
  private:
   // Documentation says:
-  // 1) Please call this from the same thread from which InitAPI() has been called (use a dedicated
-  // thread
-  //    if necessary). This avoids problems in initializing the dependent Common RunTime C
-  //    libraries.
+  // Please call this from the same thread from which InitAPI() has been called (use a dedicated
+  // thread if necessary). This avoids problems in initializing the dependent Common RunTime C
+  // libraries.
   static void RunInitOrShutdown(std::function<void(int)> work) {
     static ThreadPool s_thread_pool_(1, 0, false, "S3ClientManager");
     s_thread_pool_.AddWork(std::move(work));