How to fix "Variables can not be used inside bindings" error in Drools ?

Issue

• When defining a new rule (like the following) in BRMS, it throws the latter given error message.

Rule:

package com.sample

import com.sample.Test;

rule "Test"
    when
           Test($firstVal : map.get("key"), $secondVal: map.get($firstVal))
    then
           System.out.println("Fired rule: Test");
    end

Stacktrace:

Variables can not be used inside bindings. Variable [$firstVal] is being used in binding 'map.get($firstVal)' : [Rule name='Test']

java.lang.IllegalArgumentException: Could not parse knowledge.
    at com.sample.Test.readKnowledgeBase(Test.java:41)
    at com.sample.Test.main(Test.java:22)
        ...

How to fix this rule?