diff --git a/Jint.Tests/Runtime/ClassTests.cs b/Jint.Tests/Runtime/ClassTests.cs
index b644451425..970e61aa1a 100644
--- a/Jint.Tests/Runtime/ClassTests.cs
+++ b/Jint.Tests/Runtime/ClassTests.cs
@@ -1,3 +1,5 @@
+using Jint.Runtime;
+
 namespace Jint.Tests.Runtime;
 
 public class ClassTests
@@ -45,4 +47,33 @@ get doubleWidth () {
         Assert.Equal(10, engine.Evaluate("board.width"));
         Assert.Equal(20, engine.Evaluate("board.doubleWidth "));
     }
+
+    [Fact]
+    public void PrivateMemberAccessOutsideOfClass()
+    {
+        var ex = Assert.Throws<JavaScriptException>(() => new Engine().Evaluate
+        (
+            """
+            class A { }
+
+            new A().#nonexistent = 1;
+            """
+        ));
+
+        Assert.Equal("Unexpected identifier '#nonexistent'", ex.Message);
+    }
+
+    [Fact]
+    public void PrivateMemberAccessAgainstUnknownMemberInConstructor()
+    {
+        var ex = Assert.Throws<JavaScriptException>(() => new Engine().Evaluate
+        (
+            """
+            class A { constructor() { #nonexistent = 2; } }
+            new A();
+            """
+        ));
+
+        Assert.Equal("Unexpected identifier '#nonexistent'", ex.Message);
+    }
 }
diff --git a/Jint/Runtime/Interpreter/Expressions/JintPrivateIdentifierExpression.cs b/Jint/Runtime/Interpreter/Expressions/JintPrivateIdentifierExpression.cs
index 77090bbf85..03764c7f37 100644
--- a/Jint/Runtime/Interpreter/Expressions/JintPrivateIdentifierExpression.cs
+++ b/Jint/Runtime/Interpreter/Expressions/JintPrivateIdentifierExpression.cs
@@ -14,13 +14,13 @@ protected override object EvaluateInternal(EvaluationContext context)
         var engine = context.Engine;
         var env = engine.ExecutionContext.PrivateEnvironment;
 
-        var privateIdentifier = env!.ResolvePrivateIdentifier(_privateIdentifierString);
+        var privateIdentifier = env?.ResolvePrivateIdentifier(_privateIdentifierString);
         if (privateIdentifier is not null)
         {
             return privateIdentifier;
         }
 
-        ExceptionHelper.ThrowReferenceError(engine.Realm, "TODO Not found!!");
+        ExceptionHelper.ThrowSyntaxError(engine.Realm, $"Unexpected identifier '#{_privateIdentifierString}'");
         return null;
     }
 }