catalog:getKeywords() seem to return a table of tables?

Hi,

I'm unfamiliar with Lua and have just started hacking with Lightroom to automate some keywording. Apologies for possibly familiar 'code resuse', but I thought the result of this script would be a list of arKeyword rather than table 

LrTasks.startAsyncTask( function()
	local all_keywords=catalog:getKeywords()
	if #all_keywords == 0 then return end
	lrItem  = 1
	varFileName = ""
	varFileNames = {}
	local ProgressScope = ProgressScope({title = "My progress bar title" ,caption = "My progress bar caption",})
	for j, a_keyword in ipairs(all_keywords) do
           varFileName =  varFileName .. type(a_keyword) .. ", "
           lrItem = lrItem + 1
	   ProgressScope:setPortionComplete(lrItem/#all_keywords)
	   ProgressScope:setCaption( lrItem .. "/" .. #all_keywords .. '  ' .. j)
	end
	ProgressScope:done()

	local f = LrView.osFactory()
	local c =
		f:row{
		bind_to_object = props,
			f:column {
				f:edit_field { value = varFileName, width_in_chars = 80, height_in_lines = 10 },
			},
		}
	dialogValue = LrDialogs.presentModalDialog(
		{
			title = "Filenames" ,
			contents = c,
		}
	)
end)

but it actually generates: 

I'm obviously doing something stupid, but what? Thank you!