diff --git a/writer/FileWriter.cpp b/writer/FileWriter.cpp
index 316a69b5..e00f586e 100644
--- a/writer/FileWriter.cpp
+++ b/writer/FileWriter.cpp
@@ -189,8 +189,12 @@ void FileWriter::AddStats(const std::optional<HDF5DataFileStatistics>& s) {
         }
         if (start_message.space_group_number)
             j["space_group_number"] = start_message.space_group_number.value();
-        if (start_message.error_value)
-            j["underload"] = start_message.error_value.value();
+        // The lowest VALID value, not the error marker. These were the same field until the marker
+        // became the value the pixels actually carry: for an unsigned image that is UINTx_MAX, and a
+        // consumer feeding this key into an XDS UNDERLOAD or a DIALS trusted range would then reject
+        // every pixel below 65535.
+        if (start_message.underload_value)
+            j["underload"] = start_message.underload_value.value();
 
         j["user_data"] = start_message.user_data;
         finalized_file_socket->Send(j.dump());