Index: Ringtoets/AssemblyTool/src/Ringtoets.AssemblyTool.IO/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResult.cs
===================================================================
diff -u -ra77ce8324ad774ff960b1292d02a51a867869d7a -r4b1149de407862f37e58b78bcf8409602ec3031f
--- Ringtoets/AssemblyTool/src/Ringtoets.AssemblyTool.IO/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResult.cs	(.../SerializableFailureMechanismSectionAssemblyResult.cs)	(revision a77ce8324ad774ff960b1292d02a51a867869d7a)
+++ Ringtoets/AssemblyTool/src/Ringtoets.AssemblyTool.IO/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResult.cs	(.../SerializableFailureMechanismSectionAssemblyResult.cs)	(revision 4b1149de407862f37e58b78bcf8409602ec3031f)
@@ -43,7 +43,7 @@
         /// <param name="probability">The probability of this assembly result.</param>
         public SerializableFailureMechanismSectionAssemblyResult(SerializableAssemblyMethod assemblyMethod,
                                                                  SerializableAssessmentType assessmentType,
-                                                                 SerializableFailureMechanismSectionCategoryGroup categoryGroup,
+                                                                 SerializableFailureMechanismSectionCategoryGroup? categoryGroup = null,
                                                                  double? probability = null) : this()
         {
             CategoryGroup = categoryGroup;
@@ -62,7 +62,7 @@
         /// Gets or sets the category group of this assembly result.
         /// </summary>
         [XmlElement(AssemblyXmlIdentifiers.FailureMechanismSectionCategoryGroup)]
-        public SerializableFailureMechanismSectionCategoryGroup CategoryGroup { get; set; }
+        public SerializableFailureMechanismSectionCategoryGroup? CategoryGroup { get; set; }
 
         /// <summary>
         /// Gets or sets the probability of this assembly result.
@@ -84,5 +84,14 @@
         {
             return Probability.HasValue;
         }
+
+        /// <summary>
+        /// Determines whether <see cref="CategoryGroup"/> should be serialized.
+        /// </summary>
+        /// <returns><c>true</c> if <see cref="CategoryGroup"/> should be serialized, <c>false</c> otherwise.</returns>
+        public bool ShouldSerializeCategoryGroup()
+        {
+            return CategoryGroup.HasValue;
+        }
     }
 }
\ No newline at end of file
Index: Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.Test/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResultTest.cs
===================================================================
diff -u -rd3b53b68914f5cead6eaf7b008168b29b2bd5754 -r4b1149de407862f37e58b78bcf8409602ec3031f
--- Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.Test/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResultTest.cs	(.../SerializableFailureMechanismSectionAssemblyResultTest.cs)	(revision d3b53b68914f5cead6eaf7b008168b29b2bd5754)
+++ Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.Test/Model/DataTypes/SerializableFailureMechanismSectionAssemblyResultTest.cs	(.../SerializableFailureMechanismSectionAssemblyResultTest.cs)	(revision 4b1149de407862f37e58b78bcf8409602ec3031f)
@@ -40,7 +40,7 @@
             // Assert
             Assert.AreEqual((SerializableAssemblyMethod) 0, assemblyResult.AssemblyMethod);
             Assert.AreEqual((SerializableAssessmentType) 0, assemblyResult.AssessmentType);
-            Assert.AreEqual((SerializableFailureMechanismSectionCategoryGroup) 0, assemblyResult.CategoryGroup);
+            Assert.IsNull(assemblyResult.CategoryGroup);
             Assert.IsNull(assemblyResult.Probability);
 
             SerializableAttributeTestHelper.AssertXmlElementAttribute<SerializableFailureMechanismSectionAssemblyResult>(
@@ -93,5 +93,27 @@
             // Assert
             Assert.AreEqual(expectedShouldSerialize, shouldSerialize);
         }
+
+        [Test]
+        [TestCase(SerializableFailureMechanismSectionCategoryGroup.IIIv, true)]
+        [TestCase(null, false)]
+        public void ShouldSerializeCategoryGroup_WithSectionCategoryGroupValues_ReturnsExpectedValue(SerializableFailureMechanismSectionCategoryGroup? categoryGroup, 
+                                                                                                     bool expectedShouldSerialize)
+        {
+            // Setup
+            var random = new Random(39);
+            var assemblyResult = new SerializableFailureMechanismSectionAssemblyResult(
+                random.NextEnumValue<SerializableAssemblyMethod>(),
+                random.NextEnumValue<SerializableAssessmentType>(),
+                categoryGroup,
+                random.NextDouble());
+
+            // Call
+            bool shouldSerialize = assemblyResult.ShouldSerializeCategoryGroup();
+
+            // Assert
+            Assert.AreEqual(expectedShouldSerialize, shouldSerialize);
+
+        }
     }
 }
\ No newline at end of file