Error: Could not generate profile.

I just installed node-profile, and when I try it on any file, I get the error

```
Could not generate profile.
```

This is the case even when I try it on a file as simple as

```
var j = 0;
for (var i = 0; i < 10000; i++) { j++; };
```

Any ideas? The log file is deleted, even if I pass a flag like `-o=profile.txt`.
