spm 无法build含有normalize.css的项目

[nomandia]

我仿照了这个官方例子
http://spmjs.io/documentation/css-and-template
spm build 时提示
/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:197 throw e; ^
Error: normalize.css not found that required by index.css
at tryFile (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/file.js:158:9)
at Function.requireFile as require
at requireFile (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:192:17)
at /home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:180:15
at Array.forEach (native)
at lookupDeps (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:154:4)
at SubClass. (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:119:7)
at Array.forEach (native)
at SubClass.Class.create._parseFiles (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:117:34)
at SubClass.Class.create._parse (/home/common/install/nodejs/node-v4.1.1-linux-x64/lib/node_modules/spm/node_modules/father/lib/package.js:98:10)

我的package.json
{
"name": "main-style",
"version": "1.0.0",
"spm": {
"dependencies": {
"normalize.css": "3.0.1"
},
"main": "index.css"
}
}
/* index.css */
@import url('normalize.css');
@import url('./a.css');

// a.css
body {background: red;}
请问这个是什么问题？