Converting Npgsqlcommand to DbCommand

I'm doing tests on methods that use database connections and I like using factory patterns, so I'm doing a factory of DbCommands. My application uses a NpgsqlCommand and the tests are going to use SQLiteCommands.

So my factory code looks like this, only the application factory code (NpgCommandFactory) is included:

public interface IDbCommandFactory{

    DbCommand Create();
}

class ClassWithMethodToTest{

    private IDbCommandFactory DbCommandFactory;            

    public ClassWithMethodToTest(){
            DbCommandFactory = new NpgCommandFactory();
    }

    private class NpgCommandFactory : IDbCommandFactory{
            private string ConnectionString = "Server=server;Port=5432;Database=database;User Id=user;Password=password;";

            public DbCommand Create(){

                var connection=new NpgsqlConnection(ConnectionString);

                connection.Open();

                var command = connection.CreateCommand();

                command.CommandText=@"
                    select
                        name
                     from
                        data";                   

                return new NpgsqlCommand(command.CommandText);                 

            }
    }
}

I'm having this error:

cannot convert from 'System.Data.Common.DbCommand' to 'Npgsql.NpgsqlCommand'

I thought this would work because NpgsqlCommand class extends the DbCommand class

public sealed class NpgsqlCommand : DbCommand, ICloneable

I've done something similar with Streams, where I returned a S3Stream (class that extends Stream class) and it worked:

public Stream Create(){         

       return new S3ReadStream(
             S3, "bucketname"
       );
}

So why when doing the same with DbCommand and NpgsqlCommand doesn't work?