diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index b5dec06..b2566cd 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -3511,4 +3511,42 @@
   delcom TestCompletion
 endfunc
 
+func Test_custom_completion()
+  func CustomComplete1(lead, line, pos)
+    return "a\nb\nc"
+  endfunc
+  func CustomComplete2(lead, line, pos)
+    return ['a', 'b']->filter({ _, val -> val->stridx(a:lead) == 0 })
+  endfunc
+  func Check_custom_completion()
+    call assert_equal('custom,CustomComplete1', getcmdcompltype())
+    return ''
+  endfunc
+  func Check_customlist_completion()
+    call assert_equal('customlist,CustomComplete2', getcmdcompltype())
+    return ''
+  endfunc
+
+  command -nargs=1 -complete=custom,CustomComplete1 Test1 echo
+  command -nargs=1 -complete=customlist,CustomComplete2 Test2 echo
+
+  call feedkeys(":Test1 \<C-R>=Check_custom_completion()\<CR>\<Esc>", "xt")
+  call feedkeys(":Test2 \<C-R>=Check_customlist_completion()\<CR>\<Esc>", "xt")
+
+  call assert_fails("call getcompletion('', 'custom')", 'E475:')
+  call assert_fails("call getcompletion('', 'customlist')", 'E475:')
+
+  call assert_equal(getcompletion('', 'custom,CustomComplete1'), ['a', 'b', 'c'])
+  call assert_equal(getcompletion('', 'customlist,CustomComplete2'), ['a', 'b'])
+  call assert_equal(getcompletion('b', 'customlist,CustomComplete2'), ['b'])
+
+  delcom Test1
+  delcom Test2
+
+  delfunc CustomComplete1
+  delfunc CustomComplete2
+  delfunc Check_custom_completion
+  delfunc Check_customlist_completion
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab