diff --git a/common/DiffractionExperiment.cpp b/common/DiffractionExperiment.cpp
index 3ade2d095..639d3399d 100644
--- a/common/DiffractionExperiment.cpp
+++ b/common/DiffractionExperiment.cpp
@@ -1843,9 +1843,11 @@ DiffractionExperiment &DiffractionExperiment::GeomRefinementAlgorithm(GeomRefine
 }
 
 gemmi::CrystalSystem DiffractionExperiment::GetCrystalSystem() const {
+    // Without a space group nothing is known of the cell's symmetry: triclinic leaves every angle of
+    // the given cell free, where any other system would hold some of them at 90 degrees.
     auto sg = GetGemmiSpaceGroup();
     if (!sg)
-        return gemmi::CrystalSystem::Monoclinic;
+        return gemmi::CrystalSystem::Triclinic;
 
     return sg->crystal_system();
 }
diff --git a/image_analysis/IndexAndRefine.cpp b/image_analysis/IndexAndRefine.cpp
index b814fbcf0..c741d0c32 100644
--- a/image_analysis/IndexAndRefine.cpp
+++ b/image_analysis/IndexAndRefine.cpp
@@ -380,6 +380,19 @@ IndexAndRefine::IndexingOutcome IndexAndRefine::DetermineLatticeAndSymmetry(Data
                 else if (!de_novo)
                     outcome.lattice_candidate = latt;
                 // else: de-novo + symmetry mismatch -> leave unset, frame is not indexed
+            } else if (!de_novo && experiment.GetUnitCell()
+                       && latt.GetUnitCell().is_close(*experiment.GetUnitCell(),
+                                                      experiment.GetIndexingSettings().GetUnitCellDistTolerance(),
+                                                      experiment.GetIndexingSettings().GetUnitCellAngleTolerance_deg())) {
+                // A cell with no space group: FFBIDX returned the lattice on the given cell's axes, and
+                // it stays on them, so that every frame is in one setting. Nothing is known of the
+                // symmetry, so it is refined with every angle free.
+                outcome.symmetry = LatticeMessage{
+                    .centering = 'P',
+                    .niggli_class = 0,
+                    .crystal_system = gemmi::CrystalSystem::Triclinic
+                };
+                outcome.lattice_candidate = latt;
             } else {
                 auto sym_result = LatticeSearch(latt);
                 outcome.symmetry = LatticeMessage{
diff --git a/tests/DiffractionExperimentTest.cpp b/tests/DiffractionExperimentTest.cpp
index 930bc9ad1..e5db38e08 100644
--- a/tests/DiffractionExperimentTest.cpp
+++ b/tests/DiffractionExperimentTest.cpp
@@ -354,7 +354,7 @@ TEST_CASE("DiffractionExperiment_ZMQPreview", "[DiffractionExperiment]") {
 TEST_CASE("DiffractionExperiment_SpaceGroup", "[DiffractionExperiment]") {
     DiffractionExperiment x;
     REQUIRE(!x.GetGemmiSpaceGroup());
-    REQUIRE(x.GetCrystalSystem() == gemmi::CrystalSystem::Monoclinic);
+    REQUIRE(x.GetCrystalSystem() == gemmi::CrystalSystem::Triclinic);
     REQUIRE_THROWS(x.SpaceGroupNumber(-1));
     REQUIRE_THROWS(x.SpaceGroupNumber(500));
     REQUIRE_NOTHROW(x.SpaceGroupNumber(96));