diff --git a/src/components/message.js b/src/components/message.js
index 58868c8..89e1238 100644
--- a/src/components/message.js
+++ b/src/components/message.js
@@ -28,7 +28,7 @@ var Message = function (options) {
 }
 
 if (
- options.zIndex
+ 'zIndex' in options
 && typeof options.zIndex === 'number'
 && options.zIndex> -1
 ) {
@@ -64,7 +64,7 @@ var Message = function (options) {
 ? nextZIndex++
 : optsZIndex
 if (
- global.top
+ 'top' in global
 && typeof global.top === 'number'
 ) {
 instance.dom.style.top = `${global.top}px`
@@ -76,7 +76,7 @@ var Message = function (options) {
 Message.config = function (options) {
 global = options || {}
 if (
- global.zIndex
+ 'zIndex' in global
 && typeof global.zIndex === 'number'
 && global.zIndex> -1
 ) {
</div><div class="naked_ctrl">
<form action="/index.cgi/larger-text" method="get" name="gate">
<p><a href="http://altstyle.alfasado.net">AltStyle</a> によって変換されたページ <a href="https://patch-diff.githubusercontent.com/raw/vue-multiple/message/pull/1.diff">(-&gt;オリジナル)</a>
/ <label>アドレス: <input type="text" name="naked_post_url" value="https://patch-diff.githubusercontent.com/raw/vue-multiple/message/pull/1.diff" size="22" /></label> <label>モード: <select name="naked_post_mode">
<option value="default">デフォルト</option>
<option value="speech">音声ブラウザ</option>
<option value="ruby">ルビ付き</option>
<option value="contrast">配色反転</option>
<option value="larger-text" selected="selected">文字拡大</option>
<option value="mobile">モバイル</option>
</select>
<input type="submit" value="表示" />
</p>
</form>
</div>