diff --git a/src/modules/Elsa.Workflows.Core/Activities/SetVariable.cs b/src/modules/Elsa.Workflows.Core/Activities/SetVariable.cs
index ba99e893d..689e847e6 100644
--- a/src/modules/Elsa.Workflows.Core/Activities/SetVariable.cs
+++ b/src/modules/Elsa.Workflows.Core/Activities/SetVariable.cs
@@ -105,7 +105,7 @@ public class SetVariable : CodeActivity
         var variable = context.ExpressionExecutionContext.EnumerateVariablesInScope().FirstOrDefault(x => x.Id == variableId);
      
         if (variable == null)
-            throw new($"Variable '{variableId}' not found.");
+            throw new InvalidOperationException($"Variable '{variableId}' not found.");
         
         var value = context.Get(Value);
         variable.Set(context, value);
diff --git a/test/unit/Elsa.Activities.UnitTests/Helpers/ActivityTestHelper.cs b/test/unit/Elsa.Activities.UnitTests/Helpers/ActivityTestHelper.cs
index d01f77a29..a17ab01f9 100644
--- a/test/unit/Elsa.Activities.UnitTests/Helpers/ActivityTestHelper.cs
+++ b/test/unit/Elsa.Activities.UnitTests/Helpers/ActivityTestHelper.cs
@@ -60,7 +60,6 @@ public static class ActivityTestHelper
         services.AddSingleton<IActivityDescriber, ActivityDescriber>();
         services.AddSingleton<IPropertyDefaultValueResolver, PropertyDefaultValueResolver>();
         services.AddSingleton<IActivityFactory, ActivityFactory>();
-        services.AddSingleton<IPropertyDefaultValueResolver, PropertyDefaultValueResolver>();
         services.AddSingleton<IPropertyUIHandlerResolver, PropertyUIHandlerResolver>();
         services.AddSingleton<IActivityRegistry, ActivityRegistry>();
         services.AddScoped<IActivityRegistryLookupService, ActivityRegistryLookupService>();
diff --git a/test/unit/Elsa.Activities.UnitTests/Primitives/SetVariableTests.cs b/test/unit/Elsa.Activities.UnitTests/Primitives/SetVariableTests.cs
index 0adc16ffa..7b94781ef 100644
--- a/test/unit/Elsa.Activities.UnitTests/Primitives/SetVariableTests.cs
+++ b/test/unit/Elsa.Activities.UnitTests/Primitives/SetVariableTests.cs
@@ -37,7 +37,7 @@ public class SetVariableTests
     {
         // Arrange
         var variable = new Variable<string?>("myVar", "initial value", "myVar");
-        var setVariable = new SetVariable<string?>(variable, new Input<string?>(default(string)));
+        var setVariable = new SetVariable<string?>(variable, new Input<string?>(null));
 
         // Act
         var context = await ActivityTestHelper.ExecuteActivityAsync(setVariable);