Accessing field from dissector from post dissector

edit

Hi! Wireshark and Lua newbie here. I am trying to use a post dissector to dissect Bluetooth LE GATT data. The data has already been decode by the built in dissector as btatt.value and I figured I could further dissect btatt.value.

However, the code below doesn't work. I keep getting this error message:

calling 'add' on bad self (string expected, got userdata)

-- Source code below:

ble_gatt_value = Field.new("btatt.value")

trivial_proto = Proto("trivial","Trivial Postdissector")

gattval_field = ProtoField.bytes("trivial.gatt_value","GATT Value", base.DASH)

trivial_proto.fields = {gattval_field}

function trivial_proto.dissector(buffer,pinfo,tree)

    local src_gatt_val = ble_gatt_value()

    if src_gatt_val then 
        --print(src_gatt_val)
        local subtree = tree:add(trivial_proto, src_gatt_val, "Trivial Protocol Data")
        subtree:add(gattval_field,src_gatt_val)

    end
end

register_postdissector(trivial_proto)

What am I doing wrong? I have seen several examples but they all used the function argument buffer(x,y) with tree:add()

Thanks in advance.