Help with SetBackdrop?

[Coote]

I've never quite used SetBackdrop before, and typically just use CreateTexture if I'm making a background frame from scratch. I've looked through several different sources (WPanels, WoWPedia, WoWProgramming), to see how it's done, and this looks about right, but the frame doesn't even load.

If I replace the whole SetBackdrop() part with CreateTexture, it shows perfectly where I've placed it.

local cht = CreateFrame("Frame", "ChatBGFrame", UIParent)
cht:SetFrameLevel(1)
cht:SetHeight(64)
cht:SetWidth(128)
cht:SetFrameStrata("BACKGROUND")
cht:SetPoint("CENTER")
cht:SetBackdrop({
    bgFile = bg, 
    edgeFile = border, 
    tile = false, tileSize = 0, edgeSize = edgesize, 
    insets = { left = 2, right = 2, top = 4, bottom = 4 }
})
cht:SetBackdropColor(setcolor(backdrop))
cht:SetBackdropBorderColor(setcolor(bordercolor))
cht:SetAlpha(1)
end