Hm, I can not reproduce your problem. The following code is working for me.

```groovy
    def buildNumber(int buildNumber) {
        this.buildNumber(Integer.toString(buildNumber))
    }

    def buildNumber(String buildNumber) {
        ensureFirst()
        selectedSelector = 'SpecificBuild'
        this.buildNumber = buildNumber
    }
```

I didn't read the Groovy Language Specification, so I'm not sure how it is supposed to work. But avoid abbreviations if possible. In this case it would be better to use `number` as parameter name.