diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index 376d82f..eb89a15 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -2429,4 +2429,26 @@
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_completefunc_first_call_complete_add()
+  new
+
+  func Complete(findstart, base) abort
+    if a:findstart
+      let col = col('.')
+      call complete_add('#')
+      return col - 1
+    else
+      return []
+    endif
+  endfunc
+
+  set completeopt=longest completefunc=Complete
+  " This used to cause heap-buffer-overflow
+  call assert_fails('call feedkeys("ifoo#\<C-X>\<C-U>", "xt")', 'E840:')
+
+  delfunc Complete
+  set completeopt& completefunc&
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable