Check fields attributes using reflection

I'm trying to find a fields in a class has a Obsolete attribute ,

What I have done is , but even thought the type has an obselete attribute its not found during iteration :

public bool Check(Type type)
{
    FieldInfo[] fields = type.GetFields(BindingFlags.NonPublic |  BindingFlags.Instance);

  foreach (var field in fields)
  { 
     if (field.GetCustomAttribute(typeof(ObsoleteAttribute), false) != null)
     {
        return true
     }
  }
}

EDIT :

class MyWorkflow: : WorkflowActivity
{
 [Obsolete("obselset")]
 public string ConnectionString { get; set; }
}

and using it like this , Check(typeof(MyWorkflow))