Drools : Can't get guava getOnlyElement to work

Question

The following rule doesn't compile :

dialect "mvel"

import drools.DroolsPojo
import function com.google.common.`collect`.Iterables.getOnlyElement;

rule "rule1"
when 
    $dpojo : DroolsPojo(answers!=null)  
    $list : List() from collect ( String(length > 0) from $dpojo.answers )

    $testObject : String() from getOnlyElement($list)            // <---- ** The error**

    // ... more constraints
then
    System.out.println("testObject:  " + $testObject)
end

I already found out that it's getOnlyElement() that causes the error. I'm using drools version 5.1

Why does it not work? How can I fix it? Any help is welcome. Thanks in advance.

Error :

java.lang.AssertionError: Unable to determine the used declarations. 
[Error: not a statement, or badly formed structure] 
[Near : {... Unknown ....}]
             ^ 
[Line: 1, Column: 0] : [Rule name='rule1']