Why can't you call a number value?

I am writing a script to switch between pages for a Spell Book. When I try to switch pages, the code I think should work throws an error saying:

Players.smash1975.PlayerGui.SpellBook.RightFrame.Back.Handler:68: attempt to call a number value

The line which is throwing the error is:

button.Parent.SpellsL.Text = getPage(page,0)

The function "getPage" is:

function getPage(page,side)
	if tonumber(page) >= -1000 then
		-- Page is a number (Is hardcoded so always should be)
		local p = pages
		for x=1,#p,1 do
			if side == 1 then
				if p[x] == (page+math.floor(side)(page-1)) then
					-- Found correct page
					local ret = ""
					local c = p[x+1]
					for i=1,#c,1 do -- Convert list to string
						ret = ret..c[i].."\n"
					end
					return ret -- return string
				end
			else
				if p[x] == (page) then
					-- Found correct page
					local ret = ""
					local c = p[x+1]
					for i=1,#c,1 do -- Convert list to string
						ret = ret..c[i].."\n"
					end
					return ret -- return string
				end
			end
		end	
	end
	return "" -- Something went wrong, return empty string
end

The error is being throw when I try to call "page" to alter the position in the list that I get the string from. (Line 7)