diff --git a/lidar_pipeline/dtm.py b/lidar_pipeline/dtm.py
index eb8f4be..be7e613 100644
--- a/lidar_pipeline/dtm.py
+++ b/lidar_pipeline/dtm.py
@@ -803,7 +803,12 @@ def create_dtm_fast(las_file, basename, dtm_dir, resolution, force=False,
         if bare_earth and source_laz is not None:
             min_grid = _min_return_grid(source_laz, width, height,
                                         (min_x, min_y, max_x, max_y))
-            lower = ~np.isnan(min_grid) & (min_grid < dtm)
+            # Cellules sans sol mesuré (NaN) : le retour le plus bas devient
+            # la mesure — sinon la comparaison NaN est fausse et la cellule
+            # retombe sur l'interpolation en fin de passe.
+            has_min = ~np.isnan(min_grid)
+            lower = has_min & (min_grid < dtm)
+            lower |= has_min & np.isnan(dtm)
             dtm = np.where(lower, min_grid, dtm)
             logger.info(f"    Sol nu : {int(lower.sum()):,} cellules raménées au retour le plus bas")