diff --git a/src/RulesEngine/HelperFunctions/Constants.cs b/src/RulesEngine/HelperFunctions/Constants.cs
index 488602a..a6a3ae5 100644
--- a/src/RulesEngine/HelperFunctions/Constants.cs
+++ b/src/RulesEngine/HelperFunctions/Constants.cs
@@ -17,6 +17,7 @@ namespace RulesEngine.HelperFunctions
         public const string OPERATOR_NULL_ERRMSG = "Operator can not be null";
         public const string OPERATOR_INCORRECT_ERRMSG = "Operator {PropertyValue} is not allowed";
         public const string RULE_NAME_NULL_ERRMSG = "Rule Name can not be null";
+        public const string OPERATOR_RULES_ERRMSG = "Cannot use Rules field when Operator is null";
         public const string LAMBDA_EXPRESSION_EXPRESSION_NULL_ERRMSG = "Expression cannot be null or empty when RuleExpressionType is LambdaExpression";
         public const string LAMBDA_EXPRESSION_OPERATOR_ERRMSG = "Cannot use Operator field when RuleExpressionType is LambdaExpression";
         public const string LAMBDA_EXPRESSION_RULES_ERRMSG = "Cannot use Rules field when RuleExpressionType is LambdaExpression";
diff --git a/src/RulesEngine/Validators/RuleValidator.cs b/src/RulesEngine/Validators/RuleValidator.cs
index c14156e..8ce4314 100644
--- a/src/RulesEngine/Validators/RuleValidator.cs
+++ b/src/RulesEngine/Validators/RuleValidator.cs
@@ -39,7 +39,7 @@ namespace RulesEngine.Validators
         {
             When(c => c.Operator == null && c.RuleExpressionType == RuleExpressionType.LambdaExpression, () => {
                 RuleFor(c => c.Expression).NotEmpty().WithMessage(Constants.LAMBDA_EXPRESSION_EXPRESSION_NULL_ERRMSG);
-                RuleFor(c => c.Rules).Empty().WithMessage(Constants.LAMBDA_EXPRESSION_RULES_ERRMSG);
+                RuleFor(c => c.Rules).Empty().WithMessage(Constants.OPERATOR_RULES_ERRMSG);
             });
         }
 
diff --git a/test/RulesEngine.UnitTest/RuleValidationTest.cs b/test/RulesEngine.UnitTest/RuleValidationTest.cs
new file mode 100644
index 0000000..d8dca9f
--- /dev/null
+++ b/test/RulesEngine.UnitTest/RuleValidationTest.cs
@@ -0,0 +1,98 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using Newtonsoft.Json;
+using RulesEngine.HelperFunctions;
+using RulesEngine.Models;
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Dynamic;
+using System.Linq;
+using System.Text.Json.Serialization;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace RulesEngine.UnitTest
+{
+    [ExcludeFromCodeCoverage]
+    public class RuleValidationTest
+    {
+        [Fact]
+        public async Task NullExpressionithLambdaExpression_ReturnsExepectedResults()
+        {
+            var workflow = GetNullExpressionithLambdaExpressionWorkflow();
+            var reSettings = new ReSettings { };
+            RulesEngine rulesEngine = new RulesEngine();
+
+            Func<Task> action = () => {
+                new RulesEngine(workflow, reSettings: reSettings);
+                return Task.CompletedTask;
+            };
+
+            Exception ex = await Assert.ThrowsAsync<Exceptions.RuleValidationException>(action);
+
+            Assert.Contains(Constants.LAMBDA_EXPRESSION_EXPRESSION_NULL_ERRMSG, ex.Message);
+
+        }
+
+        [Fact]
+        public async Task NestedRulesWithMissingOperator_ReturnsExepectedResults()
+        {
+            var workflow = GetEmptyOperatorWorkflow();
+            var reSettings = new ReSettings { };
+            RulesEngine rulesEngine = new RulesEngine();
+
+            Func<Task> action = () => {
+                new RulesEngine(workflow, reSettings: reSettings);
+                return Task.CompletedTask;
+            };
+
+            Exception ex = await Assert.ThrowsAsync<Exceptions.RuleValidationException>(action);
+
+            Assert.Contains(Constants.OPERATOR_RULES_ERRMSG, ex.Message);
+
+        }
+
+        private Workflow[] GetNullExpressionithLambdaExpressionWorkflow()
+        {
+            return new[] {
+                new Workflow {
+                    WorkflowName = "NestedRulesTest",
+                    Rules = new Rule[] {
+                        new Rule {
+                            RuleName = "TestRule",
+                            RuleExpressionType = RuleExpressionType.LambdaExpression,
+                        }
+                    }
+                }
+            };
+        }
+
+        private Workflow[] GetEmptyOperatorWorkflow()
+        {
+            return new[] {
+                new Workflow {
+                    WorkflowName = "NestedRulesTest",
+                    Rules = new Rule[] {
+                        new Rule {
+                            RuleName = "AndRuleTrueFalse",
+                            Expression = "true == true",
+                            Rules = new Rule[] {
+                                new Rule{
+                                    RuleName = "trueRule1",
+                                    Expression = "input1.TrueValue == true",
+                                },
+                                new Rule {
+                                    RuleName = "falseRule1",
+                                    Expression = "input1.TrueValue == false"
+                                }
+
+                            }
+                        }
+                    }
+                }
+            };
+        }
+    }
+}