diff --git a/writer/FileWriter.cpp b/writer/FileWriter.cpp
index 25ae1c38..213b060e 100644
--- a/writer/FileWriter.cpp
+++ b/writer/FileWriter.cpp
@@ -120,11 +120,9 @@ void FileWriter::CloseOldFiles(uint64_t current_image_number) {
 }
 
 std::vector<HDF5DataFileStatistics> FileWriter::Finalize() {
-
-    if (master_file) {
-        std::lock_guard<std::mutex> lock(hdf5_mutex);
+    std::lock_guard<std::mutex> lock(hdf5_mutex);
+    if (master_file)
         master_file.reset();
-    }
 
     for (uint64_t f = 0; f < files.size(); ++f) {
         if (files[f] && !closed_files.contains(f))