A single rule in a DRL file:

rule "minute ticker"
timer( int: 1m 1m )
when
then
System.out.println( "Tick" );
end

causes

xception in thread "main" java.lang.ArrayStoreException
at java.lang.System.arraycopy(Native Method)
at java.util.ArrayList.toArray(ArrayList.java:328)
at org.drools.compiler.PackageBuilder.getErrors(PackageBuilder.java:1274)
at org.drools.builder.impl.KnowledgeBuilderImpl.getErrors(KnowledgeBuilderImpl.java:70)
at event.Main.compilePackage(Main.java:72)
at event.Main.main(Main.java:219)