Index: Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.TestUtil/SerializableAttributeTestHelper.cs
===================================================================
diff -u -re17ab48b3abda85cd6d9cb33e6a9f507e70fdff3 -rd3b53b68914f5cead6eaf7b008168b29b2bd5754
--- Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.TestUtil/SerializableAttributeTestHelper.cs	(.../SerializableAttributeTestHelper.cs)	(revision e17ab48b3abda85cd6d9cb33e6a9f507e70fdff3)
+++ Ringtoets/AssemblyTool/test/Ringtoets.AssemblyTool.IO.TestUtil/SerializableAttributeTestHelper.cs	(.../SerializableAttributeTestHelper.cs)	(revision d3b53b68914f5cead6eaf7b008168b29b2bd5754)
@@ -20,6 +20,7 @@
 // All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Xml.Serialization;
 using Core.Common.Util.Reflection;
@@ -37,11 +38,16 @@
         /// </summary>
         /// <param name="type">The type to assert.</param>
         /// <param name="typeName">The expected XML type name.</param>
-        /// <exception cref="AssertionException">Thrown when the <paramref name="typeName"/>
-        /// does not match with the actual attribute.</exception>
+        /// <exception cref="AssertionException">Thrown when:
+        /// <list type="bullet">
+        /// <item><paramref name="typeName"/> does not match with the actual attribute;</item>
+        /// <item>multiple attributes of type <see cref="XmlTypeAttribute"/> were found.</item>
+        /// </list>
+        /// </exception>
         public static void AssertXmlTypeAttribute(Type type, string typeName)
         {
-            var attribute = (XmlTypeAttribute) type.GetCustomAttributes(typeof(XmlTypeAttribute), false).Single();
+            var attribute = (XmlTypeAttribute) type.GetCustomAttributes(typeof(XmlTypeAttribute), false).SingleOrDefault();
+            Assert.IsNotNull(attribute);
             Assert.AreEqual(typeName, attribute.TypeName);
         }
 
@@ -55,12 +61,14 @@
         /// <param name="namespaceUrl">The expected XML namespace url.</param>
         /// <exception cref="AssertionException">Thrown when:
         /// <list type="bullet">
-        /// <item>the <see cref="XmlElementAttribute"/> could not be found, or multiple attributes are defined;</item>
+        /// <item>the <see cref="XmlElementAttribute"/> could not be found</item>
+        /// <item>multiple attributes of type <see cref="XmlElementAttribute"/> were found;</item>
         /// <item>the <paramref name="elementName"/> or <paramref name="namespaceUrl"/> do not match
         /// with the actual attribute.</item>
         /// </list>
         /// </exception>
         public static void AssertXmlElementAttribute<T>(string propertyName, string elementName, string namespaceUrl = null)
+            where T : class 
         {
             XmlElementAttribute attribute = GetPropertyAttribute<T, XmlElementAttribute>(propertyName);
             Assert.AreEqual(elementName, attribute.ElementName);
@@ -77,12 +85,14 @@
         /// <param name="namespaceUrl">The expected XML namespace url.</param>
         /// <exception cref="AssertionException">Thrown when:
         /// <list type="bullet">
-        /// <item>the <see cref="XmlElementAttribute"/> could not be found, or multiple attributes are defined;</item>
-        /// <item>the <paramref name="elementName"/> or <paramref name="namespaceUrl"/> do not match
-        /// with the actual attribute.</item>
+        /// <item>the <see cref="XmlElementAttribute"/> could not be found;</item>
+        /// <item>multiple attributes of type <see cref="XmlAttributeAttribute"/> were found;</item>
+        /// <item>the actual attribute does not match with the given <paramref name="elementName"/>
+        /// and <paramref name="namespaceUrl"/>.</item>
         /// </list>
         /// </exception>
         public static void AssertXmlAttributeAttribute<T>(string propertyName, string elementName, string namespaceUrl = null)
+            where T : class 
         {
             XmlAttributeAttribute attribute = GetPropertyAttribute<T, XmlAttributeAttribute>(propertyName);
             Assert.AreEqual(elementName, attribute.AttributeName);
@@ -91,9 +101,9 @@
 
         private static TAttribute GetPropertyAttribute<TObject, TAttribute>(string propertyName)
         {
-            TAttribute attribute = TypeUtils.GetPropertyAttributes<TObject, TAttribute>(propertyName).SingleOrDefault();
-            Assert.IsNotNull(attribute);
-            return attribute;
+            IEnumerable<TAttribute> attributes = TypeUtils.GetPropertyAttributes<TObject, TAttribute>(propertyName);
+            Assert.AreEqual(1, attributes.Count());
+            return attributes.Single();
         }
     }
 }
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