diff --git a/mne/viz/evoked_field.py b/mne/viz/evoked_field.py
index 2138f800136..839259ee117 100644
--- a/mne/viz/evoked_field.py
+++ b/mne/viz/evoked_field.py
@@ -279,8 +279,9 @@ def _prepare_surf_map(self, surf_map, color, alpha):
         current_data = data_interp(self._current_time)
 
         # Make a solid surface
+        surf = surf_map["surf"]
         if self._units == "mm":
-            surf = deepcopy(surf_map["surf"])
+            surf = deepcopy(surf)
             surf["rr"] *= 1000
         map_vmax = self._vmax.get(surf_map["kind"])
         if map_vmax is None: