Unique Validator: Conversion failed when converting the nvarchar value to data type int (.Net MVC)

I am having an issue when using the Unique Validator on Create

.Field(new Field("Serial")
   .Validator(Validation.Unique(new ValidationOpts
    {
         Message = "An asset with this Serial Number already exists."
     }))
)

When I try to create an item with just a number like 1000. I receive the conversion error:

Conversion failed when converting the nvarchar value 'JXRM1D1' to data type int.

This is the serial number column. But I have another column (Tag) that I use Unique on that i receive:

Conversion failed when converting the nvarchar value '000AT0001252' to data type int.

When I try and create them with a number followed by an letter, 1000z, it works.

I have Serial as well as the Tag set to public string in my view model.

Any help would be appreciated.

Thank you.