"unable to resolve method using strict-mode" error during rule compilation when using Boolean attribute in Fact model

Issue

When Fact model includes boolean attribute defined like this:

 private Boolean gpa;

        public Boolean isGpa() {
                         return gpa;
        }

the compilation of a rule, which uses this attribute in LHS, like this:

when
Fact(gpa == true)

will fail with following error:

[Error: unable to resolve method using strict-mode: Fact.gpa()]
[Near : {... gpa == true ....}]