Can't unset a numeric key

`renv -e staging -f json -a npm-website unset 3`

```
/usr/local/lib/node_modules/renv/renv.js:189
return this._del(key.split(".").join("/"), { recursive: true }).nodeify(cb);
^

TypeError: key.split is not a function
at REnv.del (/usr/local/lib/node_modules/renv/renv.js:189:26)
at /usr/local/lib/node_modules/renv/renv.js:186:20
at arrayMap (/usr/local/lib/node_modules/renv/node_modules/lodash/index.js:1406:25)
at Function.map (/usr/local/lib/node_modules/renv/node_modules/lodash/index.js:6710:14)
at REnv.del (/usr/local/lib/node_modules/renv/renv.js:185:26)
at Object.commands.unset (/usr/local/lib/node_modules/renv/bin/renv.js:143:12)
at Object. (/usr/local/lib/node_modules/renv/bin/renv.js:301:20)
at Module._compile (module.js:413:34)
at Object.Module._extensions..js (module.js:422:10)
at Module.load (module.js:357:32)
at Function.Module._load (module.js:314:12)
at Function.Module.runMain (module.js:447:10)
at startup (node.js:139:18)
at node.js:999:3
```