diff --git a/newsfragments/XXX.bugfix b/newsfragments/XXX.bugfix
new file mode 100644
index 0000000000..f0601f2564
--- /dev/null
+++ b/newsfragments/XXX.bugfix
@@ -0,0 +1 @@
+``dials.reindex``: When reindexing against a reference, use the best cell when determining lattice symmetry.
diff --git a/src/dials/command_line/reindex.py b/src/dials/command_line/reindex.py
index 1be752f48d..d46fc22210 100644
--- a/src/dials/command_line/reindex.py
+++ b/src/dials/command_line/reindex.py
@@ -159,8 +159,8 @@ def run(args=None):
                 )
 
             reference_crystal = reference_experiments.crystals()[0]
-            reference_crystal.unit_cell = determine_best_unit_cell(
-                reference_experiments
+            reference_crystal.set_unit_cell(
+                determine_best_unit_cell(reference_experiments)
             )
 
     if params.reference.reflections is not None: