diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index fe6fdfe..cb3a4d1 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -106,6 +106,7 @@ jobs:
         run: >
           dotnet test LibLouis.NET.Test/LibLouis.NET.Test.csproj
           --configuration Release
+          -warnaserror
           -p:RestoreAdditionalProjectSources=${{ github.workspace }}/local-feed
 
   # Managed packages. Split from native-linux because building LibLouis.NET resolves
diff --git a/LibLouis.NET.Test/LiblouisHeapTests.cs b/LibLouis.NET.Test/LiblouisHeapTests.cs
index aaa7509..c32270a 100644
--- a/LibLouis.NET.Test/LiblouisHeapTests.cs
+++ b/LibLouis.NET.Test/LiblouisHeapTests.cs
@@ -17,6 +17,10 @@ public unsafe class LiblouisHeapTests
 {
     private static readonly string[] Tables = ["da-dk-g26.ctb", "da-dk-g16-markers.ctb"];
 
+    // The C runtimes a mingw-built liblouis.dll can import free() from: msvcrt.dll for the gcc
+    // builds, ucrtbase.dll for llvm-mingw.
+    private static readonly string[] WindowsRuntimes = ["msvcrt.dll", "ucrtbase.dll"];
+
     private static string[] TablePaths() =>
         [.. Tables.Select(t => Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "tables", t))];
 
@@ -123,7 +127,7 @@ public void FindImport_FindsFreeInTheLiblouisWeShip()
         nint module = NativeLibrary.Load("liblouis", typeof(LibLouis).Assembly, DllImportSearchPath.SafeDirectories);
         nint free = LiblouisHeap.FindImport((byte*)module, "free"u8);
 
-        nint[] runtimeFrees = [.. new[] { "msvcrt.dll", "ucrtbase.dll" }
+        nint[] runtimeFrees = [.. WindowsRuntimes
             .Select(name => NativeLibrary.TryLoad(name, out nint runtime) ? NativeLibrary.GetExport(runtime, "free") : 0)];
 
         Assert.NotEqual(0, free);
diff --git a/build/build_managed_packages.sh b/build/build_managed_packages.sh
index 08c3ca8..c141b16 100755
--- a/build/build_managed_packages.sh
+++ b/build/build_managed_packages.sh
@@ -21,7 +21,8 @@ if [ -n "${NUGET_LOCAL_FEED:-}" ]; then
     restore_args="-p:RestoreAdditionalProjectSources=$NUGET_LOCAL_FEED"
 fi
 
-dotnet build --configuration Release $restore_args "$REPO_ROOT/LibLouis.NET.slnx"
+# -warnaserror keeps the build warning free: a warning fails the packages rather than shipping.
+dotnet build --configuration Release -warnaserror $restore_args "$REPO_ROOT/LibLouis.NET.slnx"
 
 if [ -z "${SKIP_TESTS:-}" ]; then
     dotnet test --configuration Release --no-build "$REPO_ROOT/LibLouis.NET.slnx"