diff --git a/tests/Smdn.Text.Ondulish/Smdn.Text.Ondulish/Translator.cs b/tests/Smdn.Text.Ondulish/Smdn.Text.Ondulish/Translator.cs
index 715aa06..3c93cba 100644
--- a/tests/Smdn.Text.Ondulish/Smdn.Text.Ondulish/Translator.cs
+++ b/tests/Smdn.Text.Ondulish/Smdn.Text.Ondulish/Translator.cs
@@ -115,7 +115,7 @@ public void Translate_ToTextWriter_OutputNull([Values(true, false)] bool convert
   {
     using var t = Create();
 
-    Assert.Throws<ArgumentNullException>(() => t.Translate(input: null!, convertKatakanaToNarrow: convertKatakanaToNarrow, output: null!));
+    Assert.Throws<ArgumentNullException>(() => t.Translate(input: string.Empty, convertKatakanaToNarrow: convertKatakanaToNarrow, output: null!));
   }
 
   [TestCase("オンドゥル", "ｵﾝﾄﾞｩﾙ")]