diff --git a/reader/HDF5MetadataSource.cpp b/reader/HDF5MetadataSource.cpp
index 61076f08..95bfc19a 100644
--- a/reader/HDF5MetadataSource.cpp
+++ b/reader/HDF5MetadataSource.cpp
@@ -611,7 +611,10 @@ HDF5MetadataSource::OpenResult HDF5MetadataSource::Open(const std::string &filen
         if (master_file->Exists("/entry/instrument/attenuator"))
             dataset->experiment.AttenuatorTransmission(
                 master_file->GetOptFloat("/entry/instrument/attenuator/attenuator_transmission"));
-        dataset->experiment.TotalFlux(master_file->GetOptFloat("/entry/instrument/beam/total_flux"));
+        auto total_flux = master_file->GetOptFloat("/entry/instrument/beam/total_flux");
+        if (total_flux.has_value() && total_flux.value() < 0)
+            total_flux.reset();   // negative value is an "unknown flux" sentinel; treat as absent
+        dataset->experiment.TotalFlux(total_flux);
 
         if (master_file->Exists("/entry/azint") && master_file->Exists("/entry/azint/bin_to_q")) {
             HDF5DataSet bin_to_q_dataset(*master_file, "/entry/azint/bin_to_q");