diff --git a/src/AndreGoepel.Core/Result.cs b/src/AndreGoepel.Core/Result.cs
index e235bd8..087f1f4 100644
--- a/src/AndreGoepel.Core/Result.cs
+++ b/src/AndreGoepel.Core/Result.cs
@@ -1,3 +1,5 @@
+using System.Diagnostics.CodeAnalysis;
+
 namespace AndreGoepel.Core;
 
 /// <summary>
@@ -6,10 +8,12 @@ namespace AndreGoepel.Core;
 /// </summary>
 public record Result
 {
+    [MemberNotNullWhen(false, nameof(Error))]
     public bool IsSuccess { get; init; }
 
     public string? Error { get; init; }
 
+    [MemberNotNullWhen(true, nameof(Error))]
     public bool IsFailure => !IsSuccess;
 
     public static Result Ok() => new() { IsSuccess = true };
diff --git a/tests/AndreGoepel.Core.Tests/Result.Tests.cs b/tests/AndreGoepel.Core.Tests/Result.Tests.cs
index 7c0b216..34d1d31 100644
--- a/tests/AndreGoepel.Core.Tests/Result.Tests.cs
+++ b/tests/AndreGoepel.Core.Tests/Result.Tests.cs
@@ -51,4 +51,42 @@ public void FailOfT_Called_ReturnsFailureWithNoValue()
         Assert.Equal(default, result.Value);
         Assert.Equal("invalid input", result.Error);
     }
+
+    // No null-forgiving operators (!) below and no explicit null checks on Error —
+    // this only compiles under TreatWarningsAsErrors (CS8602) if the
+    // [MemberNotNullWhen] attributes on IsSuccess/IsFailure are correct.
+
+    [Fact]
+    public void IsFailure_True_NarrowsErrorToNonNull()
+    {
+        // Arrange
+        var result = Result.Fail("boom");
+
+        // Act
+        var length = result.IsFailure ? result.Error.Length : 0;
+
+        // Assert
+        Assert.Equal(4, length);
+    }
+
+    [Fact]
+    public void IsSuccess_False_NarrowsErrorToNonNullInElseBranch()
+    {
+        // Arrange
+        var result = Result.Fail("boom");
+
+        // Act
+        string message;
+        if (result.IsSuccess)
+        {
+            message = "ok";
+        }
+        else
+        {
+            message = result.Error;
+        }
+
+        // Assert
+        Assert.Equal("boom", message);
+    }
 }