diff --git a/image_analysis/IndexAndRefine.cpp b/image_analysis/IndexAndRefine.cpp
index ebe85b0c..7b9563f8 100644
--- a/image_analysis/IndexAndRefine.cpp
+++ b/image_analysis/IndexAndRefine.cpp
@@ -101,16 +101,27 @@ IndexAndRefine::IndexingOutcome IndexAndRefine::DetermineLatticeAndSymmetry(Data
         auto latt = indexer_result.lattice[0];
         if (latt.CalcVolume() > 1.0) {
             auto sg = experiment.GetGemmiSpaceGroup();
+            const auto algorithm = experiment.GetIndexingAlgorithm();
+            const bool de_novo = (algorithm == IndexingAlgorithmEnum::FFT
+                                  || algorithm == IndexingAlgorithmEnum::FFTW);
 
-            // If space group and cell provided => always enforce symmetry in refinement
-            // If space group not provided => guess symmetry
+            // If space group and cell provided => enforce that symmetry in refinement.
+            // If not => detect the symmetry from the lattice.
             if (sg && experiment.GetUnitCell()) {
+                // FFBIDX returns the lattice already in the reference setting, so use it
+                // as-is. De-novo indexers (FFT/FFTW) return an arbitrarily-oriented primitive
+                // cell that must first be reduced to the conventional setting, else enforcing
+                // the crystal system on mis-assigned axes rejects every frame. Centering is
+                // taken from the user's space group; niggli_class is left unassigned (0) - it
+                // is a property of the primitive cell incl. centering, which LatticeSearch
+                // cannot recover from a cell the user may have given centered (e.g. C2). A
+                // proper primitive-cell indexing path (CrystFEL-style) is deferred.
                 outcome.symmetry = LatticeMessage{
                     .centering = sg->centring_type(),
                     .niggli_class = 0,
                     .crystal_system = sg->crystal_system()
                 };
-                outcome.lattice_candidate = latt;
+                outcome.lattice_candidate = de_novo ? LatticeSearch(latt).conventional : latt;
             } else {
                 auto sym_result = LatticeSearch(latt);
                 outcome.symmetry = LatticeMessage{