请求一个图像，但nginx返回200和一个空白的响应头

当浏览器（Windows IE8,9和Windows Chrome（20）和Mac Chrome（20））请求一组图像时，某些图像请求会返回304并正确显示，但其他图像请求会返回200OK并显示一个空白的响应标头，来自页面的图像。

Request URL: http://example.com/statics/images/b_9215b819d9fc2500feb1ec8e12d41f98.jpg Request Method: GET Status Code: 200 OK Request Headersview source Accept: */* Accept-Charset: GBK,utf-8;q=0.7,*;q=0.3 Accept-Encoding: gzip,deflate,sdch Accept-Language: zh-CN,zh;q=0.8 Connection: keep-alive Cookie: remember_user_token=BAhbB1sGaQLoHkkiIiQyYSQxMCRNRjE5UWMydVBUajBYcExHMy93dWdPBjoGRVQ%3D--5d1166be5cabacc37c4b8d246105f4cb9bd35b7b; CNZZDATA3722561=cnzz_eid=41795721-1343110343-&ntime=1343360172&cnzz_a=16&retime=1343360179980&sin=none&ltime=1343360179980&rtime=3; _sparrow_session=f9afb2f3c4f87635eca43fc1a94abc85 Host: example.com If-Modified-Since: Fri, 20 Jul 2012 21:06:01 GMT If-None-Match: "b65cefa739e8ac4fb62bc00304962d3c" Referer: http://example.com/guide User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11 QIHU 360EE Response Headersview source x-content-type-options: nosniff 

帮帮我〜谢谢。