Evaluate return value from Jenkins shared library in unit test with jenkins-spock

So I have a scripted shared pipeline library in my vars folder that returns some value at the end:

def call() {
  def a = 3
  //does stuff
  return a
}

Now I try to test it like this:

def "example test"() {
  when:
  def result = scriptUnderTest.call()
  then:
  result == 3
}

This won't work since result will always be null. I have written quite some tests with Jenkins-Spock for different scenarios already, so the basic mechanisms are clear. But what am I missing in this case?

The catch could be on the //does stuff part.
Here is a working test of a step that returns a value:

Inside sayHello.groovy we define a step that gets the stdout from a shell and concatenates to it:

def call() {
    def msg = sh (
        returnStdout: true,
        script: "echo Hello"
    )
    msg += " World"
    return msg
}

Inside sayHelloSpec.groovy we write our unit test and check the return value:

import com.homeaway.devtools.jenkins.testing.JenkinsPipelineSpecification

public class sayHelloSpec extends JenkinsPipelineSpecification {

    def "sayHello returns expected value" () {
        def sayHello = null

        setup:
            sayHello = loadPipelineScriptForTest("vars/sayHello.groovy")
            // Stub the sh step to return Hello
            getPipelineMock("sh")(_) >> {
                return "Hello"
            }

        when:
            def msg = sayHello()

        then:
            msg == "Hello World"
    }

}