diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index f61ff53..770890e 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -738,6 +738,8 @@
   call CheckDefFailure("let x = {'a': xxx}", 'E1001:')
   call CheckDefFailure("let x = {xxx: 8}", 'E1001:')
   call CheckDefFailure("let x = #{a: 1, a: 2}", 'E721:')
+  call CheckDefFailure("let x += 1", 'E1020:')
+  call CheckDefFailure("let x = x + 1", 'E1001:')
   call CheckDefExecFailure("let x = g:anint.member", 'E715:')
   call CheckDefExecFailure("let x = g:dict_empty.member", 'E716:')
 enddef