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)