Ruleflow execution failure due to org.mvel2.ScriptRuntimeException

Issue

We are seeing ruleflow execution failure with the following MVEL exception:

Caused by: org.mvel2.ScriptRuntimeException: expected Boolean; but found: java.lang.String
        at org.mvel2.util.ExecutionStack.popBoolean(ExecutionStack.java:125) [mvel2-2.2.8.Final-redhat-1.jar:]
        at org.mvel2.MVELRuntime.execute(MVELRuntime.java:101) [mvel2-2.2.8.Final-redhat-1.jar:]
        at
...
        at org.mvel2.MVEL.executeExpression(MVEL.java:929) [mvel2-2.2.8.Final-redhat-1.jar:]
        at org.drools.core.base.mvel.MVELConsequence.evaluate(MVELConsequence.java:110) [drools-core-6.5.0.Final-redhat-9.jar:6.5.0.Final-redhat-9]
        at org.drools.core.common.DefaultAgenda.fireActivation(DefaultAgenda.java:1054) [drools-core-6.5.0.Final-redhat-9.jar:6.5.0.Final-redhat-9]
        ... 83 more