diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index e09f913..4ebf65a 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -184,6 +184,21 @@
   v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected string but got number', 3)
 enddef
 
+def Test_add_const()
+  var lines =<< trim END
+      const l = [1, 2]
+      add(l, 3)
+  END
+  v9.CheckDefFailure(lines, 'E1307: Argument 1: Trying to modify a const list<number>')
+
+  lines =<< trim END
+      const b = 0z0102
+      add(b,  0z03)
+  END
+  v9.CheckDefFailure(lines, 'E1307: Argument 1: Trying to modify a const blob')
+enddef
+
+
 def Test_and()
   v9.CheckDefAndScriptFailure(['and("x", 0x2)'], ['E1013: Argument 1: type mismatch, expected number but got string', 'E1210: Number required for argument 1'])
   v9.CheckDefAndScriptFailure(['and(0x1, "x")'], ['E1013: Argument 2: type mismatch, expected number but got string', 'E1210: Number required for argument 2'])
@@ -1181,6 +1196,29 @@
   v9.CheckScriptFailure(lines, 'E1001: Variable not found: m')
 enddef
 
+def Test_extend_const()
+  var lines =<< trim END
+      const l = [1, 2]
+      extend(l, [3])
+  END
+  v9.CheckDefFailure(lines, 'E1307: Argument 1: Trying to modify a const list<number>')
+
+  lines =<< trim END
+      const d = {a: 1, b: 2}
+      extend(d, {c: 3})
+  END
+  v9.CheckDefFailure(lines, 'E1307: Argument 1: Trying to modify a const dict<number>')
+
+  # item in a for loop is const
+  lines =<< trim END
+      var l: list<dict<any>> = [{n: 1}]
+      for item in l
+        item->extend({x: 2})
+      endfor
+  END
+  v9.CheckDefFailure(lines, 'E1307: Argument 1: Trying to modify a const dict<any>')
+enddef
+
 def Test_extendnew()
   assert_equal([1, 2, 'a'], extendnew([1, 2], ['a']))
   assert_equal({one: 1, two: 'a'}, extendnew({one: 1}, {two: 'a'}))
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 54b4ea1..fd07fbf 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -2471,10 +2471,11 @@
   lines =<< trim END
       var l: list<dict<any>> = [{n: 1}]
       for item: dict<number> in l
-        item->extend({s: ''})
+        var d = {s: ''}
+        d->extend(item)
       endfor
   END
-  v9.CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected dict<number> but got dict<string>')
+  v9.CheckDefExecAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected dict<string> but got dict<number>')
 
   lines =<< trim END
       for a in range(3)