diff --git a/src/Jiaowu.Api/Infrastructure/Files/DownloadFileName.cs b/src/Jiaowu.Api/Infrastructure/Files/DownloadFileName.cs
index ff65cf7..86cd563 100644
--- a/src/Jiaowu.Api/Infrastructure/Files/DownloadFileName.cs
+++ b/src/Jiaowu.Api/Infrastructure/Files/DownloadFileName.cs
@@ -5,6 +5,11 @@ namespace Jiaowu.Api.Infrastructure.Files;
 /// </summary>
 public static class DownloadFileName
 {
+    // Do not use Path.GetInvalidFileNameChars(): its result is platform-specific,
+    // while downloaded names must remain usable on Windows regardless of where
+    // the API is built or hosted.
+    private static readonly char[] WindowsInvalidFileNameChars = ['<', '>', ':', '"', '/', '\\', '|', '?', '*'];
+
     public static string Export(string title, string extension, params string?[] parts) =>
         Build(title, extension, includeTimestamp: true, parts);
 
@@ -33,9 +38,8 @@ public static class DownloadFileName
     private static string Clean(string? value)
     {
         if (string.IsNullOrWhiteSpace(value)) return string.Empty;
-        var invalid = Path.GetInvalidFileNameChars();
         var cleaned = new string(value.Trim().Select(character =>
-            invalid.Contains(character) ? '-' : character).ToArray());
+            WindowsInvalidFileNameChars.Contains(character) || char.IsControl(character) ? '-' : character).ToArray());
         return cleaned.Trim('.', ' ');
     }
 
diff --git a/tests/Jiaowu.Api.Tests/CourseSelectionsControllerTests.cs b/tests/Jiaowu.Api.Tests/CourseSelectionsControllerTests.cs
index f15cbae..d01220e 100644
--- a/tests/Jiaowu.Api.Tests/CourseSelectionsControllerTests.cs
+++ b/tests/Jiaowu.Api.Tests/CourseSelectionsControllerTests.cs
@@ -110,12 +110,14 @@ public sealed class CourseSelectionsControllerTests
             .OrderBy(x => x.UserId)
             .ToListAsync();
         Assert.Equal(2, resultNotifications.Count);
-        var selectedNotification = Assert.Single(resultNotifications
-            .Where(x => x.UserId == data.EnrolledUserId));
+        var selectedNotification = Assert.Single(
+            resultNotifications,
+            x => x.UserId == data.EnrolledUserId);
         Assert.Contains("最终选中 1 门", selectedNotification.Content);
         Assert.Contains("《程序设计基础》", selectedNotification.Content);
-        var waitlistNotification = Assert.Single(resultNotifications
-            .Where(x => x.UserId == data.FirstWaiterUserId));
+        var waitlistNotification = Assert.Single(
+            resultNotifications,
+            x => x.UserId == data.FirstWaiterUserId);
         Assert.Contains("本轮未选中课程", waitlistNotification.Content);
         Assert.Contains("候补未成功 1 门", waitlistNotification.Content);
         Assert.Equal(NotificationCategory.CourseSelection, waitlistNotification.Category);
diff --git a/tests/Jiaowu.Api.Tests/GradeAnalysisWordReportGeneratorTests.cs b/tests/Jiaowu.Api.Tests/GradeAnalysisWordReportGeneratorTests.cs
index d619ad7..72ef19c 100644
--- a/tests/Jiaowu.Api.Tests/GradeAnalysisWordReportGeneratorTests.cs
+++ b/tests/Jiaowu.Api.Tests/GradeAnalysisWordReportGeneratorTests.cs
@@ -43,16 +43,19 @@ public sealed class GradeAnalysisWordReportGeneratorTests
         Assert.True(bytes.Length > 10_000);
         using var stream = new MemoryStream(bytes);
         using var document = WordprocessingDocument.Open(stream, false);
-        var text = document.MainDocumentPart!.Document.InnerText;
+        var mainDocumentPart = document.MainDocumentPart
+            ?? throw new InvalidOperationException("Generated document has no main document part.");
+        var text = mainDocumentPart.Document?.InnerText
+            ?? throw new InvalidOperationException("Generated document has no document body.");
         Assert.Contains("成绩分析报告", text);
         Assert.Contains("同课程教学班对比", text);
         Assert.Contains("各范围基准", text);
         Assert.Contains("历年成绩趋势", text);
         Assert.Contains("行政班组成", text);
         Assert.Contains("计科一班", text);
-        Assert.Equal(3, document.MainDocumentPart.ImageParts.Count());
-        Assert.Equal(2, document.MainDocumentPart.HeaderParts.Count());
-        Assert.Equal(2, document.MainDocumentPart.FooterParts.Count());
+        Assert.Equal(3, mainDocumentPart.ImageParts.Count());
+        Assert.Equal(2, mainDocumentPart.HeaderParts.Count());
+        Assert.Equal(2, mainDocumentPart.FooterParts.Count());
     }
 
     [Fact]
@@ -76,10 +79,13 @@ public sealed class GradeAnalysisWordReportGeneratorTests
         Assert.True(bytes.Length > 8_000);
         using var stream = new MemoryStream(bytes);
         using var document = WordprocessingDocument.Open(stream, false);
-        var text = document.MainDocumentPart!.Document.InnerText;
+        var mainDocumentPart = document.MainDocumentPart
+            ?? throw new InvalidOperationException("Generated document has no main document part.");
+        var text = mainDocumentPart.Document?.InnerText
+            ?? throw new InvalidOperationException("Generated document has no document body.");
         Assert.Contains("科目总体成绩分析报告", text);
         Assert.Contains("学生来源学院对比", text);
         Assert.Contains("教学班横向比较", text);
-        Assert.Single(document.MainDocumentPart.ImageParts);
+        Assert.Single(mainDocumentPart.ImageParts);
     }
 }
diff --git a/tests/Jiaowu.Api.Tests/OfficialDocumentTests.cs b/tests/Jiaowu.Api.Tests/OfficialDocumentTests.cs
index 67ef210..dbd4af9 100644
--- a/tests/Jiaowu.Api.Tests/OfficialDocumentTests.cs
+++ b/tests/Jiaowu.Api.Tests/OfficialDocumentTests.cs
@@ -28,8 +28,9 @@ public sealed class OfficialDocumentTests
         string parameterName)
     {
         var constructor = Assert.Single(requestType.GetConstructors());
-        var parameter = Assert.Single(constructor.GetParameters()
-            .Where(x => x.Name == parameterName));
+        var parameter = Assert.Single(
+            constructor.GetParameters(),
+            x => x.Name == parameterName);
         Assert.NotEmpty(parameter.GetCustomAttributes(typeof(ValidationAttribute), false));
 
         var property = requestType.GetProperty(parameterName);