Asp.net mvc 4 已忽略自定义validationattribute错误消息

我有以下自定义ValidationAttribute，其工作方式与所需的validation属性相同，但我接受将其值连接到错误消息的属性的名称。但是，错误消息总是以“需要[fieldName]字段”的形式出现

怎么了

public class RequiredFieldWithValueOnErrorMessage : ValidationAttribute
    {
// a little portuguese lesson :P
// propriedade = property
// mensagemDeErro = errorMessage

        public RequiredFieldWithValueOnErrorMessage (String propriedade, String mensagemDeErro)
        {
            if (!String.IsNullOrEmpty(mensagemDeErro))
            {
                this.ErrorMessage = mensagemDeErro;
            }
            else
            {
                this.ErrorMessageResourceName = "MensagemCampoRequerido";
                this.ErrorMessageResourceType = typeof(MensagensInterfaceUsuario);
            }

            Propriedade = propriedade;
        }

        private String Propriedade { get; set; }

        public override string FormatErrorMessage(string name)
        {
            return String.Format(ErrorMessage, name);
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            Object instance = validationContext.ObjectInstance;
            Type type = instance.GetType();
            Object propertyValue = type.GetProperty(Propriedade).GetValue(instance, null);
            if (value == null || value.ToString() == "0" || String.IsNullOrWhiteSpace(value.ToString()))
            {
                return new ValidationResult(FormatErrorMessage(propertyValue.ToString()));
            }
            return ValidationResult.Success;
        }
    }

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我发现的一件事是，如果我将正在验证的属性的类型更改为Int64？（nullable int），它可以工作。为什么？