Gradle: unexpected UP-TO-DATE task while output file is missing

build.gradle

apply plugin: 'java'

ext {
    dest = file( "${buildDir}/generated-files" )
}

task generate(type:JavaExec, dependsOn: [classes]) {
    def destFile = new File(dest, "foo.txt")
    outputs.file destFile
    main 'org.foo.bar.FooMain'
    classpath sourceSets.main.runtimeClasspath
    args destFile
    doFirst {
        dest.mkdirs()
    }
}

FooMain.java

package org.foo.bar;

import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;

public class FooMain
{
    public static void main( String[] args )
        throws FileNotFoundException, UnsupportedEncodingException
    {
        boolean skip = false;
        if ( !skip )
        {
            System.out.println( "Writing file" );
            PrintWriter writer = new PrintWriter( args[0], "UTF-8" );
            writer.println( "Hello World!" );
            writer.close();
        }
    }
}