diff --git a/src/Radical/ChangeTracking/Change Management/Change.cs b/src/Radical/ChangeTracking/Change Management/Change.cs
index d45f95bc..4b0f0328 100644
--- a/src/Radical/ChangeTracking/Change Management/Change.cs	
+++ b/src/Radical/ChangeTracking/Change Management/Change.cs	
@@ -21,7 +21,7 @@ public abstract class Change<T> : IChange<T>
         /// The callback to invoke in order to
         /// commit the cached value.
         /// </summary>
-        protected CommitCallback<T> CommitCallback { get; }
+        protected CommitCallback<T>? CommitCallback { get; }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Change&lt;T&gt;"/> class.
@@ -31,7 +31,7 @@ public abstract class Change<T> : IChange<T>
         /// <param name="rejectCallback">The reject callback.</param>
         /// <param name="commitCallback">The commit callback.</param>
         /// <param name="description">The description.</param>
-        protected Change(object owner, T valueToCache, RejectCallback<T> rejectCallback, CommitCallback<T> commitCallback, string description)
+        protected Change(object owner, T valueToCache, RejectCallback<T> rejectCallback, CommitCallback<T>? commitCallback, string description)
         {
             Ensure.That(owner).Named("owner").IsNotNull();
             Ensure.That(rejectCallback).Named("rejectCallback").IsNotNull();
diff --git a/src/Radical/Model/ChangeTracking/Properties/PropertyValueChange (Generic).cs b/src/Radical/Model/ChangeTracking/Properties/PropertyValueChange (Generic).cs
index 23e3cfb1..4e13341f 100644
--- a/src/Radical/Model/ChangeTracking/Properties/PropertyValueChange (Generic).cs	
+++ b/src/Radical/Model/ChangeTracking/Properties/PropertyValueChange (Generic).cs	
@@ -45,7 +45,7 @@ public PropertyValueChange(object owner, string propertyName, T value, RejectCal
         /// <param name="restoreCallback">The restore callback.</param>
         /// <param name="commitCallback">The commit callback.</param>
         /// <param name="description">The description.</param>
-        public PropertyValueChange(object owner, string propertyName, T value, RejectCallback<T> restoreCallback, CommitCallback<T> commitCallback, string description)
+        public PropertyValueChange(object owner, string propertyName, T value, RejectCallback<T> restoreCallback, CommitCallback<T>? commitCallback, string description)
             : base(owner, value, restoreCallback, commitCallback, description)
         {
             PropertyName = propertyName;
diff --git a/src/Radical/Radical.csproj b/src/Radical/Radical.csproj
index 4da727b7..700351bb 100644
--- a/src/Radical/Radical.csproj
+++ b/src/Radical/Radical.csproj
@@ -2,6 +2,8 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <LangVersion>9</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup>