diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
index f1ae0f6237e..38e014d59e4 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
@@ -22,12 +22,12 @@ public void Ctor_String_String(string xmlNamespace, string clrNamespace)
     [Fact]
     public void Ctor_NullXmlNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsDefinitionAttribute(null, "clrNamespace"));
+        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsDefinitionAttribute(null!, "clrNamespace"));
     }
 
     [Fact]
     public void Ctor_NullClrNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("clrNamespace", () => new XmlnsDefinitionAttribute("xmlNamespace", null));
+        Assert.Throws<ArgumentNullException>("clrNamespace", () => new XmlnsDefinitionAttribute("xmlNamespace", null!));
     }
 }
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
index 4cdc46a8a92..f8b2c744725 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
@@ -21,12 +21,12 @@ public void Ctor_String_String(string xmlNamespace, string prefix)
     [Fact]
     public void Ctor_NullXmlNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsPrefixAttribute(null, "prefix"));
+        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsPrefixAttribute(null!, "prefix"));
     }
 
     [Fact]
     public void Ctor_NullPrefix_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("prefix", () => new XmlnsPrefixAttribute("xmlNamespace", null));
+        Assert.Throws<ArgumentNullException>("prefix", () => new XmlnsPrefixAttribute("xmlNamespace", null!));
     }
 }