Insert via EntityFramework "IDENTITY_INSERT is set to OFF" error

I have several models and for some reason one of them has the following error:

Cannot insert explicit value for identity column in table 'tbl_boleto' when IDENTITY_INSERT is set to OFF.

I remember that I have already entered a record, but now I can not anymore.

    var NewBoleto = new Boleto
    {
        ClienteId = 3,
        DataVencimento = DateTime.Today,
        Valor = 4,
        DataBaixa=null,
        DataEmissao = DateTime.Now,
        DataReferencia= DateTime.Today,
        DataPagamento = null
    };

   db.Boletos.Add(NewBoleto);
   db.SaveChanges();

The model:

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace WMB.Models
{
    [Table("tbl_boleto")]
    public class Boleto
    {
        [Key]
        [Column("int_ID")]
        public int BoletoId { get; set; }

        [Column("int_IDC")]
        public int ClienteId { get; set; }

        [Column("sdt_Dataemissao")]
        public DateTime DataEmissao { get; set; }

        [Column("sdt_DataVencimento")]
        [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:yyyy-MM-dd}")]
        [Required(ErrorMessage = "Data de Vencimento é requerida")]
        [Display(Name = "Data de Vencimento")]
        [DataType(DataType.Date)]
        public DateTime DataVencimento { get; set; }


        [Column("sdt_dataPagamento")]
        public DateTime? DataPagamento { get; set; }


        [Column("sdt_dataBaixa")]
        public DateTime? DataBaixa { get; set; }

        [Column("sdt_DataReferencia")]
        [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:yyyy-MM-dd}")]
        [Display(Name = "Referência 01/mm/aaaa")]
        [DataType(DataType.Date)]
        public DateTime DataReferencia { get; set; }

        [Column("cur_valor")]
        public decimal? Valor { get; set; }

        [Column("cur_valorPago")]
        public decimal? ValorPago { get; set; }

        [Column("int_status")]
        public byte Status { get; set; }

        [Column("int_TotalMeses")]
        public byte? TotalMeses { get; set; }

        [Column("int_Tipo")]
        public byte Tipo { get; set; }

        [Column("int_ViewClient")]
        public byte? QtdVisualizacoes { get; set; }


        public virtual Cliente Cliente { get; set; }

        //[ForeignKey("BoletoId")]
        //public virtual BoletoTXT boletoTxt { get; set; }

    }
}

When looking through the SQL profile for some unknown reason for this model it is sending the PK field which is self-contained. (int_ID) For all other models it is working.

Update : I discovered the reason for the error, but I do not know why. I have another Model called BoletoTXT that has a connection with Boleto N-1

 [Table("tbl_boleto_TXT")]
    public class BoletoTXT
    {
        [Key]
        [Column("int_ID")]
        public int BoletoTXTId { get; set; }

        [Column("int_IDBoleto")]
        public int BoletoId { get; set; }

        [Column("str_Mensagem")]
        public string Menssagem { get; set; }

        [ForeignKey("BoletoId")]
        public virtual ICollection<boleto> boleto { get; set; }

    }

The problem is this line

[ForeignKey("BoletoId")]
public virtual ICollection<boleto> boleto { get; set; }

But I've used it like this on other projects and I do not remember that I made a mistake. Am I making the wrong relationship?