SLF4J groovy annotation compilation error. "log" was found in a static scope, but does not apply to a local variable, static field or class

I looked at this block of code for a good hour, trying to understand why the log is not matched at compile time. I use gradle to build and have the dependencies listed as such:

apply plugin: 'groovy'

repositories {
    mavenCentral()
}

dependencies {
    compile 'org.codehaus.groovy:groovy:2.2.1'
    compile 'org.slf4j:slf4j-api:1.7.6'
    provided 'org.projectlombok:lombok:1.12.4'
    runtime 'ch.qos.logback:logback-core:1.1.1'
    runtime 'ch.qos.logback:logback-classic:1.1.1'
    testCompile 'junit:junit:4.11'
}

My “code block” is a class with a name FilesUtilannotated with help @groovy.logging.util.SLF4Jthat contains one method staticthat uses a variable log, as the javadoc annotation suggests

import groovy.util.logging.Slf4j

import java.nio.file.FileVisitResult
import java.nio.file.Files
import java.nio.file.Path
import java.nio.file.SimpleFileVisitor
import java.nio.file.attribute.BasicFileAttributes

import static java.nio.file.FileVisitResult.CONTINUE;

@Slf4j
class FilesUtil {
    def static deleteDirectory(Path path) {
        Files.walkFileTree(path, new SimpleFileVisitor<Path>() {
            @Override
            public FileVisitResult visitFile(Path file,
                                             BasicFileAttributes attrs) throws IOException {
                log.trace("deleting file: $file")
                System.out.println("Deleting file: " + file)
                Files.delete(file);
                log.trace("deleted file: $file")
                return CONTINUE;
            }

            @Override
            public FileVisitResult postVisitDirectory(Path dir,
                                                      IOException exc) throws IOException {
                if (exc == null) {
                    log.trace("deleting directory: $dir")
                    Files.delete(dir);
                    log.trace("deleted directory: $dir")
                    return CONTINUE;
                } else {
                    throw exc;
                }
            }
        })
    }
} 

When called gradlew build(using the gradle shell) I get 4 similar errors, for example:

startup failed:
C:\Users\Macindows\IdeaProjects\corporate-git\subprojects\core\src\main\groovy\com\thenaglecode\corporategit\core\util\files\FilesUtil.groovy: 26: 
Apparent variable 'log' was found in a static scope but doesn't refer to a local variable, static field or class. Possible causes:
    You attempted to reference a variable in the binding or an instance variable from a static context.
    You misspelled a classname or statically imported field. Please check the spelling.
    You attempted to use a method 'log' but left out brackets in a place not allowed by the grammar.
    @ line 26, column 17.
               log.trace("deleting file: $file")
               ^

Where is Wally? (a mistake I cannot find ... Waldo is for you, American people)