Couldn't anymore personalize comparison method starting from drools 5.4.0

I can't write anymore (since 5.4.0) straightforward rules such as :

rule "linux kernel too old"
when
s:LinuxServer(release < "2.6.18")
then
insert(new ConfWarning(s,
"Linux kernel is too old "+s.name))
end

where release is using Release type with a dedicated comparator :

case class Release(
name:String
) extends Ordered[AnyRef] {
override def compare(that:AnyRef) : Int = that match

{ case Release(thatname) => NaturalSort.ord.compare(name, thatname) case thatname:String => NaturalSort.ord.compare(name, thatname) case _ => 1 }

}

I got the following error with the latest drools (it was ok with previous releases) :
[Error: Comparison operation requires compatible types. Found class com.orange.confxpert.model.Release and class java.lang.String]

I know that type enforcing is very important, but we should have a way to choose how strict must be type enforcements.

User experience is very important, saying release < "2.6.18" is quite more user friendly than saying release < Release.fromString("2.6.18"). This is an important issue, as rules may be written by non IT people.

[info] Running com.orange.confxpert.Main -a -r -v examples/xxxx/yyyyy
Unable to Analyse Expression release < "2.6.18":
[Error: Comparison operation requires compatible types. Found class xxxxx.model.Release and class java.lang.String]
[Near :

{... release < "2.6.18" ....}

]
^ : [Rule name='linux kernel too old']

[error] (run-main) java.lang.IllegalArgumentException: Could not parse knowledge.
java.lang.IllegalArgumentException: Could not parse knowledge.
at org.drools.builder.impl.KnowledgeBuilderImpl.newKnowledgeBase(KnowledgeBuilderImpl.java:73)
at zzzzzz.Main$.analyze(Main.scala:66)
at zzzzzz..Main$$anonfun$main$1.apply(Main.scala:98)
at zzzzzz..Main$$anonfun$main$1.apply(Main.scala:96)
at scala.Option.map(Option.scala:133)
at zzzzzz..Main$.main(Main.scala:96)
at zzzzzz..Main.main(Main.scala)
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)
java.lang.RuntimeException: Nonzero exit code: 1
at scala.sys.package$.error(package.scala:27)