Using named parameter only when passing integer

Currently I'm trying to work with Named Parameters using SAP Sybase SQL Anywhere 12 with dapper. The following codes runs correctly:

public class Test
{
    public int Str1
    {
        get;
        set;
    }

    public string Str2
    {
        get;
        set;
    }
}

class Program
{
    static void Main(string[] args)
    {
        using (SAConnection connection = new SAConnection("..."))
        {
            connection.Open();

            Test test = connection.Query<Test>("SELECT :Str1 as Str1, :Str2 as Str2",
                new Test() { Str1 = 35, Str2 = "42" }).FirstOrDefault();                    

            Console.WriteLine($"Str1: {test.Str1} | Str2: {test.Str2}");
            Console.ReadLine();
        }
    }
}

But when i change Str2 = "42" to some string, than i get the following exception:

Cast 42a to integer not possible

This exception is thrown when I'm using the following code:

Test test = connection.Query<Test>("SELECT :Str1 as Str1, :Str2 as Str2",
new Test() { Str1 = 35, Str2 = "42a" }).FirstOrDefault();  

Is there some known issue? This should work correctly, cause i just want to pass a string around.

Edit

Stack trace:

iAnywhere.Data.SQLAnywhere.SAException (0x80004005): Umwandeln von '42a' auf integer nicht möglich bei iAnywhere.Data.SQLAnywhere.SACommand._ExecuteReader(CommandBehavior commandBehavior, Boolean isExecuteScalar, Boolean isBeginExecuteReader) bei iAnywhere.Data.SQLAnywhere.SACommand.ExecuteDbDataReader(CommandBehavior behavior) bei System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior) bei Dapper.SqlMapper.d__611.MoveNext() bei System.Collections.Generic.List1..ctor(IEnumerable1 collection) bei System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) bei Dapper.SqlMapper.Query[T](IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Boolean buffered, Nullable1 commandTimeout, Nullable1 commandType) bei DapperSqlAnywhere.Program.Main(String[] args) in C:\Users....\DapperSqlAnywhere\Program.cs:Zeile 35.

Thanks a lot!