diff --git a/SysML2.NET.Tests/Extend/EndFeatureMembershipExtensionsTestFixture.cs b/SysML2.NET.Tests/Extend/EndFeatureMembershipExtensionsTestFixture.cs
index 8a121303..90be40c4 100644
--- a/SysML2.NET.Tests/Extend/EndFeatureMembershipExtensionsTestFixture.cs
+++ b/SysML2.NET.Tests/Extend/EndFeatureMembershipExtensionsTestFixture.cs
@@ -1,38 +1,88 @@
-// -------------------------------------------------------------------------------------------------
+// -------------------------------------------------------------------------------------------------
 // <copyright file="EndFeatureMembershipExtensionsTestFixture.cs" company="Starion Group S.A.">
-// 
+//
 //   Copyright 2022-2026 Starion Group S.A.
-// 
+//
 //   Licensed under the Apache License, Version 2.0 (the "License");
 //   you may not use this file except in compliance with the License.
 //   You may obtain a copy of the License at
-// 
+//
 //        http://www.apache.org/licenses/LICENSE-2.0
-// 
+//
 //    Unless required by applicable law or agreed to in writing, software
 //    distributed under the License is distributed on an "AS IS" BASIS,
 //    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 //    See the License for the specific language governing permissions and
 //    limitations under the License.
-// 
+//
 // </copyright>
 // ------------------------------------------------------------------------------------------------
 
 namespace SysML2.NET.Tests.Extend
 {
     using System;
-    
+
     using NUnit.Framework;
-    
+
     using SysML2.NET.Core.POCO.Core.Features;
+    using SysML2.NET.Core.POCO.Root.Elements;
+    using SysML2.NET.Core.POCO.Root.Namespaces;
+    using SysML2.NET.Exceptions;
+    using SysML2.NET.Extensions;
+
+    using Type = SysML2.NET.Core.POCO.Core.Types.Type;
 
     [TestFixture]
     public class EndFeatureMembershipExtensionsTestFixture
     {
         [Test]
-        public void ComputeOwnedMemberFeature_ThrowsNotSupportedException()
+        public void VerifyComputeOwnedMemberFeature()
         {
-            Assert.That(() => ((IEndFeatureMembership)null).ComputeOwnedMemberFeature(), Throws.TypeOf<NotSupportedException>());
+            Assert.That(() => ((IEndFeatureMembership)null).ComputeOwnedMemberFeature(), Throws.TypeOf<ArgumentNullException>());
+
+            // Empty OwnedRelatedElement → [1..1] violation: throws IncompleteModelException.
+            var endFeatureMembership = new EndFeatureMembership();
+
+            Assert.That(() => endFeatureMembership.ComputeOwnedMemberFeature(), Throws.TypeOf<IncompleteModelException>());
+
+            // Single IFeature wired via the public API → returned.
+            var owningType = new Type();
+            var feature = new Feature();
+
+            owningType.AssignOwnership(endFeatureMembership, feature);
+
+            Assert.That(endFeatureMembership.ComputeOwnedMemberFeature(), Is.SameAs(feature));
+
+            // Two IFeatures in OwnedRelatedElement → [1..1] violation: throws IncompleteModelException.
+            var twoFeatureMembership = new EndFeatureMembership();
+            var firstFeature = new Feature();
+            var secondFeature = new Feature();
+
+            ((IContainedRelationship)twoFeatureMembership).OwnedRelatedElement.Add(firstFeature);
+            ((IContainedRelationship)twoFeatureMembership).OwnedRelatedElement.Add(secondFeature);
+
+            Assert.That(() => twoFeatureMembership.ComputeOwnedMemberFeature(), Throws.TypeOf<IncompleteModelException>());
+
+            // Mixed-type owned related elements: exactly one IFeature alongside a non-IFeature (Namespace).
+            // The RequireSingleOfType<IFeature> projection MUST pick out the IFeature regardless of its position
+            // (this is the core robustness guarantee — never positionally index the unfiltered collection).
+            var mixedMembership = new EndFeatureMembership();
+            var siblingNonFeature = new Namespace();
+            var mixedFeature = new Feature();
+
+            ((IContainedRelationship)mixedMembership).OwnedRelatedElement.Add(siblingNonFeature);
+            ((IContainedRelationship)mixedMembership).OwnedRelatedElement.Add(mixedFeature);
+
+            Assert.That(mixedMembership.ComputeOwnedMemberFeature(), Is.SameAs(mixedFeature));
+
+            // OwnedRelatedElement populated with non-IFeature element(s) only → no IFeature match:
+            // [1..1] violation, throws IncompleteModelException.
+            var nonFeatureMembership = new EndFeatureMembership();
+            var nonFeatureElement = new Namespace();
+
+            ((IContainedRelationship)nonFeatureMembership).OwnedRelatedElement.Add(nonFeatureElement);
+
+            Assert.That(() => nonFeatureMembership.ComputeOwnedMemberFeature(), Throws.TypeOf<IncompleteModelException>());
         }
     }
 }
diff --git a/SysML2.NET/Extend/EndFeatureMembershipExtensions.cs b/SysML2.NET/Extend/EndFeatureMembershipExtensions.cs
index 538a5844..1e5ef201 100644
--- a/SysML2.NET/Extend/EndFeatureMembershipExtensions.cs
+++ b/SysML2.NET/Extend/EndFeatureMembershipExtensions.cs
@@ -21,13 +21,8 @@
 namespace SysML2.NET.Core.POCO.Core.Features
 {
     using System;
-    using System.Collections.Generic;
 
-    using SysML2.NET.Core.Root.Namespaces;
-    using SysML2.NET.Core.POCO.Core.Types;
-    using SysML2.NET.Core.POCO.Root.Annotations;
-    using SysML2.NET.Core.POCO.Root.Elements;
-    using SysML2.NET.Core.POCO.Root.Namespaces;
+    using SysML2.NET.Extensions;
 
     /// <summary>
     /// The <see cref="EndFeatureMembershipExtensions"/> class provides extensions methods for
@@ -44,11 +39,14 @@ internal static class EndFeatureMembershipExtensions
         /// <returns>
         /// the computed result
         /// </returns>
-        [System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
         internal static IFeature ComputeOwnedMemberFeature(this IEndFeatureMembership endFeatureMembershipSubject)
         {
-            throw new NotSupportedException("Create a GitHub issue when this method is required");
-        }
+            if (endFeatureMembershipSubject == null)
+            {
+                throw new ArgumentNullException(nameof(endFeatureMembershipSubject));
+            }
 
+            return endFeatureMembershipSubject.OwnedRelatedElement.RequireSingleOfType<IFeature>(nameof(endFeatureMembershipSubject));
+        }
     }
 }