diff --git a/.github/workflows/dotnet.yml b/.github/workflows/dotnet.yml
index 578c78cd3..400846fb7 100644
--- a/.github/workflows/dotnet.yml
+++ b/.github/workflows/dotnet.yml
@@ -13,12 +13,16 @@ on:
 
 jobs:
   build-dotnet:
-    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     timeout-minutes: 15
     steps:
     - uses: actions/checkout@v4
       with:
         fetch-depth: 0 # avoid shallow clone so nbgv can do its work.
     - uses: ./.github/actions/setup-dotnet
-    - run: dotnet build -c Release -t:build,pack
+    - run: dotnet build -c Release -t:build,pack -warnaserror
     - run: dotnet test -c Release --no-build
diff --git a/tests/MessagePack.SourceGenerator.Tests/Verifiers/CSharpSourceGeneratorVerifier`1+Test.cs b/tests/MessagePack.SourceGenerator.Tests/Verifiers/CSharpSourceGeneratorVerifier`1+Test.cs
index 74288a368..6ff839d83 100644
--- a/tests/MessagePack.SourceGenerator.Tests/Verifiers/CSharpSourceGeneratorVerifier`1+Test.cs
+++ b/tests/MessagePack.SourceGenerator.Tests/Verifiers/CSharpSourceGeneratorVerifier`1+Test.cs
@@ -2,7 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 // Uncomment the following line to write expected files to disk
-#define WRITE_EXPECTED
+// #define WRITE_EXPECTED
 
 #if WRITE_EXPECTED
 #warning WRITE_EXPECTED is fine for local builds, but should not be merged to the main branch.
diff --git a/tests/MessagePack.Tests/MessagePackWriterTests.cs b/tests/MessagePack.Tests/MessagePackWriterTests.cs
index 3f990845a..b3a02d907 100644
--- a/tests/MessagePack.Tests/MessagePackWriterTests.cs
+++ b/tests/MessagePack.Tests/MessagePackWriterTests.cs
@@ -155,7 +155,10 @@ public void WriteBinHeader()
         public void Write_ByteArray_OldSpec_AvoidsStr8(int length)
         {
             byte[] value = new byte[length];
-            Array.Fill(value, (byte)'A');
+            for (int i = 0; i < value.Length; i++)
+            {
+                value[i] = (byte)'A';
+            }
 
             var sequence = new Sequence<byte>();
             var writer = new MessagePackWriter(sequence) { OldSpec = true };