context.Set<Employee>().Where(e => e.ReportsTo.Equals(longPrm)).ToList();

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'System.UInt64' to type 'System.Nullable`1[System.Int32]'.
   at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)
   at MongoDB.Driver.Linq.Linq3Implementation.Misc.SerializationHelper.SerializeValue(IBsonSerializer serializer, Object value)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.EqualsMethodToFilterTranslator.Translate(TranslationContext context, Expression expression, Expression expression1, Expression expression2)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.EqualsMethodToFilterTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.MethodCallExpressionToFilterTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateUsingQueryOperators(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.Translate(TranslationContext context, Expression expression, Boolean exprOk)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateLambda(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.WhereMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.ExpressionToPipelineTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.AsMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.ExpressionToPipelineTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider`1 provider, Expression expression, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.Execute()
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.GetEnumerator()
   at MongoDB.EntityFrameworkCore.Query.QueryingEnumerable`2.Enumerator.MoveNextHelper()
   at MongoDB.EntityFrameworkCore.Query.QueryingEnumerable`2.Enumerator.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Program.<Main>$(String[] args) in /Users/arthur.vickers/code/OnYourOwnNow/OnYourOwnNow/Program.cs:line 19

using var context = new SomeDbContext();
context.Database.EnsureDeleted();

var entities = new Employee { EmployeeID = 1, ReportsTo = 1 };
context.AddRange(entities );
context.SaveChanges();

ulong longPrm = 2;
var results = context.Set<Employee>().Where(e => e.ReportsTo.Equals(longPrm)).ToList();    

foreach (var result in results)
{
    Console.WriteLine($"Found {result}");
}

public class SomeDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseMongoDB("mongodb://localhost:27017", "SomeDb3")
            .EnableSensitiveDataLogging()
            .LogTo(Console.WriteLine, LogLevel.Information);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Employee>();
    }
}

public class Employee
{
    public int EmployeeID { get; set; }

    [MaxLength(10)] [Required] public string FirstName { get; set; }

    [MaxLength(30)] public string Title { get; set; }

    [MaxLength(15)] public string City { get; set; }

    [MaxLength(15)] public string Country { get; set; }

    public int? ReportsTo { get; set; }
}