FitNesse command line: Process already terminated?. Exit Code: 1

I wanted to integrate my FitNesse tests into my gradle build:

build.gradle

task command(type:Exec) {
    try {
        commandLine = [
            'java', 
            '-cp', 
            'fitnesse-standalone-2012.12.20.gebit7.jar', 
            'fitnesseMain.FitNesseMain',
            '-c',
            'FitNesse.UserGuide?suite&format=text'
        ]
    } catch ( all ) {
        println ( "There was a failure on testing:" )
        println ( all.getMessage() )
    }
}

Calling it with "gradle command" works fine. The FitNesse server is started and if I am not mistaken all tests inside FitNesse/UserGuide are executed.

The problem is that this command never terminates. After some time after the tests are all done I can see in the command prompt (it stays like this all the time, nothing happens after that anymore):

Command prompt

Process already terminated?. Exit Code: 1
--------
59 Tests,       49 Failures     0,000 seconds.

> Building 0% > :command

What is the problem here? Why is the command not terminating?