diff --git a/app/comp/res_common.py b/app/comp/res_common.py
index 1f19fbfd..ed223271 100644
--- a/app/comp/res_common.py
+++ b/app/comp/res_common.py
@@ -629,7 +629,7 @@ class ResultatsSemestre(ResultatsCache):
             c_class = f"_{col_id}_class"
             if "col_empty" in bottom_infos["moy"].get(c_class, ""):
                 for row in rows:
-                    row[c_class] += " col_empty"
+                    row[c_class] = row.get(c_class, "") + " col_empty"
                 titles[c_class] += " col_empty"
                 for row in bottom_infos.values():
                     row[c_class] = row.get(c_class, "") + " col_empty"