diff --git a/runtime/lua/vim/lsp/completion.lua b/runtime/lua/vim/lsp/completion.lua
index ab490de4e5..4f3852adb8 100644
--- a/runtime/lua/vim/lsp/completion.lua
+++ b/runtime/lua/vim/lsp/completion.lua
@@ -1004,7 +1004,13 @@ local function trigger(bufnr, clients, ctx)
       end
 
       local result = response.result
-      if result and #(result.items or result) > 0 then
+      if type(result) == 'table' and result.items == vim.NIL then
+        error(
+          ('%s: completion response has items=null, expected CompletionItem[]'):format(
+            client and client.name or 'UNKNOWN'
+          )
+        )
+      elseif result and result ~= vim.NIL and #(result.items or result) > 0 then
         Context.isIncomplete = Context.isIncomplete or result.isIncomplete
         local encoding = client and client.offset_encoding or 'utf-16'
         local client_matches, tmp_server_start_boundary
diff --git a/test/functional/plugin/lsp/completion_spec.lua b/test/functional/plugin/lsp/completion_spec.lua
index 8bc7771d81..c8310e94c9 100644
--- a/test/functional/plugin/lsp/completion_spec.lua
+++ b/test/functional/plugin/lsp/completion_spec.lua
@@ -1182,6 +1182,24 @@ describe('vim.lsp.completion: protocol', function()
       eq({ triggerKind = 2, triggerCharacter = 'h' }, exec_lua('return _G.params.context'))
     end)
   end)
+
+  it('errors on invalid items=null in completion response #39400', function()
+    create_server('dummy', {
+      isIncomplete = false,
+      items = vim.NIL,
+    })
+    feed('ih')
+    local err = t.pcall_err(function()
+      exec_lua(function()
+        vim.api.nvim_win_set_cursor(0, { 1, 1 })
+        vim.lsp.completion.get()
+        vim.wait(1000, function()
+          return false
+        end)
+      end)
+    end)
+    t.matches('items=null', err)
+  end)
 end)
 
 describe('vim.lsp.completion: integration', function()