尝试将点数添加到我的计分器时出错。Lua

当我在创建我的lua游戏的计分器时遇到了一个错误。这里是我的代码。

score = 0

local playerScore = display.newText("Score" ..score, 0, 10, "AmericanTypewriter-Bold",          16);
playerScore:setTextColor(0, 0, 0);

playerScore.text = "Score: " .. score

function ball:touch( event )
if event.phase == "began" then
playerScore.text = playerScore.text + 1
ball:applyForce(0, -10)
return true
end
end

这里是给我错误的代码行。

playerScore.text = playerScore.text + 1

它给我的错误信息。

Attempt to perform arithmetic on field 'text' (a string value)