问题：“创建协议字段中尝试索引全局变量'base'”。

我正在尝试为 Wireshark 编写 Lua 解析器。我需要解析头字段版本 = 4 个字节（0x00000000）

我的代码：

do
    local asc_sccp = Proto("asc_sccp", "ASC Skinny Client Control Protocol")
    local f = asc_sccp.fields
    f.length = ProtoField.bytes("asc_sccp.length", "length")
    f.version = ProtoField.uint8("asc_sccp.version", "version", base.HEX, 0xC)
    function asc_sccp.init()
    end
    function asc_sccp.dissector(buffer, pinfo, tree)
        local subtree = tree:add(asc_sccp, buffer())
        local offset = 0
        pinfo.cols.protocol = asc_sccp.name
        local length = buffer(offset, 4)
        subtree:add(f.length, length)
        subtree:append_text("Data length: " .. length)
        offset = offset + 4
        local version = buffer(offset, 4)
        subtree:add(f.version, version)
        subtree:append_text("    Version: " .. version)
    end
    local tcp_table = DissectorTable.get("tcp.port")
    tcp_table:add(2000, asc_sccp)
end

我为什么会收到错误“尝试对全局变量'base'(空值)进行索引”？ 您能否帮忙，我查看了许多解析器示例，但找不到解决方案。