diff --git a/src/testdir/test_crypt.vim b/src/testdir/test_crypt.vim
index 1782d83..85ed574 100644
--- a/src/testdir/test_crypt.vim
+++ b/src/testdir/test_crypt.vim
@@ -438,4 +438,27 @@
   bwipe!
 endfunc
 
+func Test_crypt_set_key_disallow_append_subtract()
+  new Xtest4
+
+  set key=foobar
+  call assert_true(&modified)
+  setl nomodified
+
+  call assert_fails('set key-=foo', 'E474:')
+  call assert_fails('set key-=bar', 'E474:')
+  call assert_fails('set key-=foobar', 'E474:')
+  call assert_fails('set key-=test1', 'E474:')
+
+  call assert_false(&modified)
+  call assert_equal('*****', &key)
+
+  call assert_fails('set key+=test2', 'E474:')
+  call assert_fails('set key^=test3', 'E474:')
+
+  call assert_false(&modified)
+  set key=
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/testdir/test_history.vim b/src/testdir/test_history.vim
index f1c31de..bb6d671 100644
--- a/src/testdir/test_history.vim
+++ b/src/testdir/test_history.vim
@@ -244,8 +244,13 @@
 " Test for making sure the key value is not stored in history
 func Test_history_crypt_key()
   CheckFeature cryptv
+
   call feedkeys(":set bs=2 key=abc ts=8\<CR>", 'xt')
   call assert_equal('set bs=2 key= ts=8', histget(':'))
+
+  call assert_fails("call feedkeys(':set bs=2 key-=abc ts=8\<CR>', 'xt')")
+  call assert_equal('set bs=2 key-= ts=8', histget(':'))
+
   set key& bs& ts&
 endfunc
 
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 32d4910..8cd5069 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -365,13 +365,15 @@
   call feedkeys(":set spellsuggest=best,file:test_options.v\<Tab>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"set spellsuggest=best,file:test_options.vim", @:)
 
-  " Expand value for 'key'
-  set key=abcd
-  call feedkeys(":set key=\<Tab>\<C-B>\"\<CR>", 'xt')
-  call assert_equal('"set key=*****', @:)
-  call feedkeys(":set key-=\<Tab>\<C-B>\"\<CR>", 'xt')
-  call assert_equal('"set key-=*****', @:)
-  set key=
+  " Expanding value for 'key' is disallowed
+  if exists('+key')
+    set key=abcd
+    call feedkeys(":set key=\<Tab>\<C-B>\"\<CR>", 'xt')
+    call assert_equal('"set key=', @:)
+    call feedkeys(":set key-=\<Tab>\<C-B>\"\<CR>", 'xt')
+    call assert_equal('"set key-=', @:)
+    set key=
+  endif
 
   " Expand values for 'filetype'
   call feedkeys(":set filetype=sshdconfi\<Tab>\<C-B>\"\<CR>", 'xt')