How to map a read-only property in Fluent nhibernate, error in identifying setter

I have a property that should be read-only, I performed mapping via code and assign the parameter to read-only, however, it gives me an exception stating that the property setter was not found

Exit: "Could not find a setter for property 'IndexValue 'in class ... "

My IndexValue attribute, which is where the exception occurs is as follows in the class.

public class MyClass: EntityBase
{
    .
    .
    .       
    public virtual decimal CropCode{ get; set ; }
    public virtual decimal UnityDiscount { get; set; }       
    public virtual decimal IndexValue { get; set; }
   }

I also tried to remove the virtual one and did not succeed.

My mapping was done as follows:

public MapMyClass()
    {
        Table("MyTable");
        CompositeId()
            .KeyProperty(c => c.Code, "CODI_EMP")
            .KeyProperty(c => c.OrCode, "PEDI_PED")       
        Map(c => c.CropCode).Column("CODI_CUL");
        Map(c => c.UnityDiscount).Column("DSAC_IPE");
        .
        .
        Map(c => c.IndexValue).ReadOnly();
    }

I've tried to do the mapping also changing to:

 Map(c => c.IndexValue).Access.ReadOnly();

and also with

  Map(c => c.IndexValue).Access.Field();

The error occurs when nhibernate tries to execute the query, I already executed the query directly in the database and everything happens normal.