diff --git a/src/Microsoft.VisualStudio.Validation/Assumes.cs b/src/Microsoft.VisualStudio.Validation/Assumes.cs
index 10f389e4..d14754a1 100644
--- a/src/Microsoft.VisualStudio.Validation/Assumes.cs
+++ b/src/Microsoft.VisualStudio.Validation/Assumes.cs
@@ -4,7 +4,6 @@
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
-using System.Globalization;
 using System.Runtime;
 using System.Runtime.CompilerServices;
 
@@ -24,7 +23,10 @@ public static partial class Assumes
     public static void NotNull<T>([ValidatedNotNull, NotNull]T? value)
         where T : class
     {
-        True(value is object);
+        if (value is null)
+        {
+            FailNotNull<T>();
+        }
     }
 
     /// <summary>
@@ -36,7 +38,10 @@ public static void NotNull<T>([ValidatedNotNull, NotNull]T? value)
     public static void NotNull<T>([ValidatedNotNull, NotNull]T? value)
         where T : struct
     {
-        True(value.HasValue);
+        if (!value.HasValue)
+        {
+            FailNotNull<T>();
+        }
     }
 
     /// <summary>
@@ -97,7 +102,10 @@ public static void NotNullOrEmpty<T>([ValidatedNotNull, NotNull]IEnumerable<T>?
     public static void Null<T>(T? value)
         where T : class
     {
-        True(value is null);
+        if (value is not null)
+        {
+            FailNull<T>();
+        }
     }
 
     /// <summary>
@@ -109,7 +117,10 @@ public static void Null<T>(T? value)
     public static void Null<T>(T? value)
         where T : struct
     {
-        False(value.HasValue);
+        if (value.HasValue)
+        {
+            FailNull<T>();
+        }
     }
 
     /// <summary>
@@ -356,6 +367,20 @@ public static Exception Fail([Localizable(false)] string? message, Exception? in
         }
     }
 
+    [DoesNotReturn]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void FailNotNull<T>()
+    {
+        Fail(Strings.FormatUnexpectedNull(typeof(T).Name));
+    }
+
+    [DoesNotReturn]
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void FailNull<T>()
+    {
+        Fail(Strings.FormatUnexpectedNonNull(typeof(T).Name));
+    }
+
     /// <summary>
     /// Helper method that formats string arguments.
     /// </summary>
diff --git a/src/Microsoft.VisualStudio.Validation/Strings.resx b/src/Microsoft.VisualStudio.Validation/Strings.resx
index 6e893c33..ff610335 100644
--- a/src/Microsoft.VisualStudio.Validation/Strings.resx
+++ b/src/Microsoft.VisualStudio.Validation/Strings.resx
@@ -144,4 +144,10 @@
   <data name="ServiceMissing" xml:space="preserve">
     <value>Cannot find an instance of the {0} service.</value>
   </data>
+  <data name="UnexpectedNull" xml:space="preserve">
+    <value>Unexpected null value of type '{0}'.</value>
+  </data>
+  <data name="UnexpectedNonNull" xml:space="preserve">
+    <value>Unexpected non-null value of type '{0}'.</value>
+  </data>
 </root>
diff --git a/test/Microsoft.VisualStudio.Validation.Tests/AssumesTests.cs b/test/Microsoft.VisualStudio.Validation.Tests/AssumesTests.cs
index 72a76ccd..e7403efa 100644
--- a/test/Microsoft.VisualStudio.Validation.Tests/AssumesTests.cs
+++ b/test/Microsoft.VisualStudio.Validation.Tests/AssumesTests.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Globalization;
 using System.Runtime.Serialization.Formatters.Binary;
 using Microsoft;
 using Moq;
@@ -9,11 +10,13 @@
 public partial class AssumesTests : IDisposable
 {
     private const string TestMessage = "Some test message.";
-    private AssertDialogSuppression suppressAssertUi = new AssertDialogSuppression();
+    private readonly AssertDialogSuppression suppressAssertUi = new();
+    private readonly OverrideCulture overrideCulture = new(CultureInfo.InvariantCulture);
 
     public void Dispose()
     {
         this.suppressAssertUi.Dispose();
+        this.overrideCulture.Dispose();
     }
 
     [Fact]
@@ -79,28 +82,40 @@ public void Fail()
     [Fact]
     public void NotNull()
     {
-        Assert.ThrowsAny<Exception>(() => Assumes.NotNull((object?)null));
+        Exception ex = Assert.ThrowsAny<Exception>(() => Assumes.NotNull((string?)null));
+
+        Assert.Equal("Unexpected null value of type 'String'.", ex.Message);
+
         Assumes.NotNull("success");
     }
 
     [Fact]
     public void NotNull_NullableStruct()
     {
-        Assert.ThrowsAny<Exception>(() => Assumes.NotNull((int?)null));
+        Exception ex = Assert.ThrowsAny<Exception>(() => Assumes.NotNull((int?)null));
+
+        Assert.Equal("Unexpected null value of type 'Int32'.", ex.Message);
+
         Assumes.NotNull((int?)5);
     }
 
     [Fact]
     public void Null()
     {
-        Assert.ThrowsAny<Exception>(() => Assumes.Null("not null"));
+        Exception ex = Assert.ThrowsAny<Exception>(() => Assumes.Null("not null"));
+
+        Assert.Equal("Unexpected non-null value of type 'String'.", ex.Message);
+
         Assumes.Null((object?)null);
     }
 
     [Fact]
     public void Null_NullableStruct()
     {
-        Assert.ThrowsAny<Exception>(() => Assumes.Null((int?)5));
+        Exception ex = Assert.ThrowsAny<Exception>(() => Assumes.Null((int?)5));
+
+        Assert.Equal("Unexpected non-null value of type 'Int32'.", ex.Message);
+
         Assumes.Null((int?)null);
     }
 
diff --git a/test/Microsoft.VisualStudio.Validation.Tests/OverrideCulture.cs b/test/Microsoft.VisualStudio.Validation.Tests/OverrideCulture.cs
new file mode 100644
index 00000000..d0ed7220
--- /dev/null
+++ b/test/Microsoft.VisualStudio.Validation.Tests/OverrideCulture.cs
@@ -0,0 +1,28 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Globalization;
+
+/// <summary>
+/// Sets a specific culture for a duration, restoring the prior culture upon disposal.
+/// </summary>
+internal sealed class OverrideCulture : IDisposable
+{
+    private readonly CultureInfo priorCulture;
+    private readonly CultureInfo priorUICulture;
+
+    public OverrideCulture(CultureInfo culture)
+    {
+        this.priorCulture = CultureInfo.CurrentCulture;
+        this.priorUICulture = CultureInfo.CurrentUICulture;
+
+        CultureInfo.CurrentCulture = culture;
+        CultureInfo.CurrentUICulture = culture;
+    }
+
+    public void Dispose()
+    {
+        CultureInfo.CurrentCulture = this.priorCulture;
+        CultureInfo.CurrentUICulture = this.priorUICulture;
+    }
+}