Framework MSBuild demands a string

Directory.Build.targets

@@ -40,7 +40,7 @@
   -->
   <Target Name="CheckNuPkgEnvEndsWithSlash"
           AfterTargets="AfterCompile">
-    <Warning Condition="'$(NUGET_PACKAGES)' != '' AND !$(NUGET_PACKAGES.EndsWith($([System.IO.Path]::DirectorySeparatorChar)))"
+    <Warning Condition="'$(NUGET_PACKAGES)' != '' AND !$(NUGET_PACKAGES.EndsWith($([System.String]::new($([System.IO.Path]::DirectorySeparatorChar)))))"
              Text="NUGET_PACKAGES should end with a slash or it will lead to editorconfig issues: $(NUGET_PACKAGES)" />
   </Target>