assertion failed when setting up screen touching

Whenever I run my program I get an error statement saying “assertion failed!”. What is Wrong? Thanks in advance!  :)    

    local function onScreenTouch(event)

        if event.x < centerX then

            if (event.phase == “began”) then

                x = event.x

                y = event.y

                tx = x

                ty = y

 

           elseif (event.phase == “moved”) then

                tx = event.x

                ty = event.y

            elseif (event.phase == “ended”) then

                x, y = 0, 0

                tx, ty = 0, 0

            end

       end

       return true

    end

    Runtime:addEventListener(“touch”, OnScreenTouch)

Oops! I fixed it!

Oops! I fixed it!