Ruby MS Access adapter

mdb.rb

require 'rubygems'
require 'win32ole'
require 'csv'

mdb_file="c:/Sites/labs/50q.bok"

class AccessDb
    attr_accessor :mdb, :connection, :data, :fields, :catalog

    def initialize(mdb=nil)
        @mdb = mdb
        @connection = nil
        @data = nil
        @fields = nil
    end

    def open
        connection_string =  'Provider=Microsoft.Jet.OLEDB.4.0;Data Source='
	#Access 2010 connection string
	# connection_string =  'Provider=Microsoft.ACE.OLEDB.12.0;Data Source='
        connection_string << @mdb
        
		@connection = WIN32OLE.new('ADODB.Connection')
		@connection.Open(connection_string)

	@catalog = WIN32OLE.new("ADOX.Catalog")
	@catalog.ActiveConnection = @connection

	# catalog.create(connection_string)
	# Source="#{@mdb_file}"
    end

    def query(sql)
        recordset = WIN32OLE.new('ADODB.Recordset')
	puts recordset.ole_methods
        recordset.Open(sql, @connection)
        @fields = []
        recordset.Fields.each do |field|
            @fields << field.Name
        end
        begin
            @data = recordset.GetRows.transpose
        rescue
            @data = []
        end
        recordset.Close
    end

    def tables
	tables = []
	@catalog.tables.each {|t| tables << t.name if t.type == "TABLE" } 
	tables
    end

    def execute(sql)
        @connection.Execute(sql)
    end

    def close
        @connection.Close
    end
end

#create empty ms access file
file = File.open(mdb_file, File::RDWR|File::CREAT)

db = AccessDb.new(mdb_file)
db.open
# db.query("Select * from b1611")
# puts db.connection.invoke "OpenSchema" , "b1611"
puts db.tables
# puts db.fields
# puts db.data

# puts db.execute "select * from tables"

# db.close

I m trying to implement a sql injection safe way to perform inserts:
sql = "INSERT INTO tblTest ([myName], [integerNumber], [isRed]) VALUES(?, ?, ?);"

ocmd = WIN32OLE.new('ADODB.Command')
ocmd.CommandType = 1
ocmd.ActiveConnection = @connection
ocmd.CommandText = sql

values.each_with_index do |value, index|
ocmd.Parameters.Item(index).Value = value
end

Unfortunetly, that fails if value is nil.

Do you know how I may pass a nil value to a parameter?