Unable to use logback.groovy, but logback.xml works

I wanted to configure Logback using Groovy DSL. The file is very simple:

import ch.qos.logback.classic.encoder.PatternLayoutEncoder
import ch.qos.logback.core.ConsoleAppender
import static ch.qos.logback.classic.Level.DEBUG
import static ch.qos.logback.classic.Level.INFO

appender("stdout", ConsoleAppender) {
    encoder(PatternLayoutEncoder) {
        pattern = "%d %p [%c] - <%m>%n"
    }
}

root(INFO, ["stdout"])

I use Gradle to build my application and run it with jettyRun. I get the following error:

Failed to instantiate [ch.qos.logback.classic.LoggerContext]
Reported exception:
org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'ch.qos.logback.core.ConsoleAppender[null]' with class 'ch.qos.logback.core.ConsoleAppender' to class 'ch.qos.logback.core.Appender'
    at  org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.castToType(DefaultTypeTransformation.java:360)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.castToType(ScriptBytecodeAdapter.java:599)
at ch.qos.logback.classic.gaffer.ConfigurationDelegate.appender(ConfigurationDelegate.groovy:119)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:90)
at org.codehaus.groovy.runtime.metaclass.MixinInstanceMetaMethod.invoke(MixinInstanceMetaMethod.java:53)
at org.codehaus.groovy.runtime.callsite.PogoMetaMethodSite$PogoMetaMethodSiteNoUnwrapNoCoerce.invoke(PogoMetaMethodSite.java:308)
at org.codehaus.groovy.runtime.callsite.PogoMetaMethodSite.callCurrent(PogoMetaMethodSite.java:52)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:46)
at org.codehaus.groovy.runtime