Index: Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/DistributionPropertiesBase.cs
===================================================================
diff -u -rd9119b8f4a74fb2a6f3ececeaea94f0e58513c52 -rf529cbef4a1bf833e87097cda48841fe077e14cc
--- Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/DistributionPropertiesBase.cs	(.../DistributionPropertiesBase.cs)	(revision d9119b8f4a74fb2a6f3ececeaea94f0e58513c52)
+++ Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/DistributionPropertiesBase.cs	(.../DistributionPropertiesBase.cs)	(revision f529cbef4a1bf833e87097cda48841fe077e14cc)
@@ -34,7 +34,7 @@
     /// Properties class for implementations of <see cref="IDistribution"/>.
     /// </summary>
     public abstract class DistributionPropertiesBase<TDistribution> : ObjectProperties<TDistribution>
-        where TDistribution : IDistribution
+        where TDistribution : class, IDistribution
     {
         private const string meanPropertyName = nameof(Mean);
         private const string standardDeviationPropertyName = nameof(StandardDeviation);
Index: Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/VariationCoefficientDistributionPropertiesBase.cs
===================================================================
diff -u -rd9119b8f4a74fb2a6f3ececeaea94f0e58513c52 -rf529cbef4a1bf833e87097cda48841fe077e14cc
--- Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/VariationCoefficientDistributionPropertiesBase.cs	(.../VariationCoefficientDistributionPropertiesBase.cs)	(revision d9119b8f4a74fb2a6f3ececeaea94f0e58513c52)
+++ Riskeer/Common/src/Riskeer.Common.Forms/PropertyClasses/VariationCoefficientDistributionPropertiesBase.cs	(.../VariationCoefficientDistributionPropertiesBase.cs)	(revision f529cbef4a1bf833e87097cda48841fe077e14cc)
@@ -34,7 +34,7 @@
     /// Properties class for implementations of <see cref="IDistribution"/>.
     /// </summary>
     public abstract class VariationCoefficientDistributionPropertiesBase<TDistribution> : ObjectProperties<TDistribution>
-        where TDistribution : IVariationCoefficientDistribution
+        where TDistribution : class, IVariationCoefficientDistribution
     {
         private const string meanPropertyName = nameof(Mean);
         private const string variationCoefficientPropertyName = nameof(CoefficientOfVariation);
Index: Riskeer/Common/test/Riskeer.Common.Forms.TestUtil/DistributionPropertiesTestHelper.cs
===================================================================
diff -u -rd9119b8f4a74fb2a6f3ececeaea94f0e58513c52 -rf529cbef4a1bf833e87097cda48841fe077e14cc
--- Riskeer/Common/test/Riskeer.Common.Forms.TestUtil/DistributionPropertiesTestHelper.cs	(.../DistributionPropertiesTestHelper.cs)	(revision d9119b8f4a74fb2a6f3ececeaea94f0e58513c52)
+++ Riskeer/Common/test/Riskeer.Common.Forms.TestUtil/DistributionPropertiesTestHelper.cs	(.../DistributionPropertiesTestHelper.cs)	(revision f529cbef4a1bf833e87097cda48841fe077e14cc)
@@ -44,7 +44,7 @@
         public static void AssertPropertiesAreReadOnly<T>(
             DistributionPropertiesBase<T> properties,
             bool meanReadOnly,
-            bool deviationReadOnly) where T : IDistribution
+            bool deviationReadOnly) where T : class, IDistribution
         {
             PropertyDescriptorCollection dynamicProperties = PropertiesTestHelper.GetAllVisiblePropertyDescriptors(properties);
             Assert.AreEqual(3, dynamicProperties.Count);
@@ -71,7 +71,7 @@
         public static void AssertPropertiesAreReadOnly<T>(
             VariationCoefficientDistributionPropertiesBase<T> properties,
             bool meanReadOnly,
-            bool variationCoefficientReadOnly) where T : IVariationCoefficientDistribution
+            bool variationCoefficientReadOnly) where T : class, IVariationCoefficientDistribution
         {
             PropertyDescriptorCollection dynamicProperties = PropertiesTestHelper.GetAllVisiblePropertyDescriptors(properties);
             Assert.AreEqual(3, dynamicProperties.Count);