For some weird reason PMD is giving this error: 

```
An internal error occurred during: "ReviewCode".
    Unable to find referenced rule UseSingleton; perhaps the rule name is mispelled?

```

this is how this rule is declared in my ruleset.xml:

```
   <rule ref="rulesets/java/design.xml/UseSingleton"/>

```

I'm using Eclipse Kepler with 4.0.6 PMD plugin version. What am I doing wrong?