Veracode throws "Technology-Specific Input Validation Problems (CWE ID 100)" for a public string property in C#

Veracode throws "Technology-Specific Input Validation Problems (CWE ID 100)" for a public string property in C#.

These are the formats I have tried already, and all give same flaw.

Option: 1

    public string MyProperty { get; set; }

Option: 2

    private string _myProperty;
    public string MyProperty
    {
        get
        {
            return _myProperty;
        }
        set
        {
            _myProperty = value;
        }
    }

Option: 3

    private string _myProperty;
    public string MyProperty
    {
        get
        {
            return _myProperty ?? string.Empty;
        }
        set
        {
            _myProperty = value;
        }
    }

Can anyone tell why?