diff --git a/src/vim9instr.c b/src/vim9instr.c
index 85a49a8..18dad11 100644
--- a/src/vim9instr.c
+++ b/src/vim9instr.c
@@ -1533,6 +1533,8 @@
     // For checking the item type we use the declared type of the list and the
     // current type of the added item, adding a string to [1, 2] is OK.
     list_type = get_decl_type_on_stack(cctx, 1);
+    if (arg_type_modifiable(list_type, 1) == FAIL)
+	return FAIL;
     item_type = get_type_on_stack(cctx, 0);
     expected = list_type->tt_member;
     if (need_type(item_type, expected, -1, 0, cctx, FALSE, FALSE) == FAIL)
@@ -1554,7 +1556,9 @@
 {
     type_T	*item_type;
 
-    // Caller already checked that blob_type is a blob.
+    // Caller already checked that blob_type is a blob, check it is modifiable.
+    if (arg_type_modifiable(get_decl_type_on_stack(cctx, 1), 1) == FAIL)
+	return FAIL;
     item_type = get_type_on_stack(cctx, 0);
     if (need_type(item_type, &t_number, -1, 0, cctx, FALSE, FALSE) == FAIL)
 	return FAIL;