diff --git a/src/lipyphilic/analysis/memb_thickness.py b/src/lipyphilic/analysis/memb_thickness.py
index 83ab30b..7979f37 100644
--- a/src/lipyphilic/analysis/memb_thickness.py
+++ b/src/lipyphilic/analysis/memb_thickness.py
@@ -285,7 +285,7 @@ def _single_frame(self):
         self.results.memb_thickness[self._frame_index] = thickness
 
         if self._return_surface:
-            self.memb_thickness_grid[self._frame_index] = upper_surface - lower_surface
+            self.memb_thickness_grid[self._frame_index] = upper_surface - lower_surface if self.n_bins > 1 else thickness
 
     def _interpolate(self, surface):
         """Interpolate the leaflet intrinsic surface.