Mockk Missing calls inside every { ... } block

I'm stuck trying to mock some stuff with mockk:

I have the following setup on gradle

root:
  |-- App (just a sample app for the SDK)
  |-- SDK (SDK we develop) << apply plugin: 'com.android.library'
       |-- SDKimpl.kt
  |-- Foo (wrapper around a .jar library) << apply plugin: 'com.android.library'
       |-- Foo.kt

So I'm writing an androidTest for the SDK and trying to mock Foo.kt. There's nothing unusual about Foo class, just direct class Foo(private val someParams) {

So using androidTestImplementation "io.mockk:mockk-android:1.8.13" the mock goes:

val mock: Foo = mockk()
// val mock: Foo = mockkClass(Foo::class) // also tried this
every { mock.getData() } returns listOf("1", "2", "3")

I'm always getting the following crash:

io.mockk.MockKException: Missing calls inside every { ... } block.
at io.mockk.impl.recording.states.StubbingState.checkMissingCalls(StubbingState.kt:14)
at io.mockk.impl.recording.states.StubbingState.recordingDone(StubbingState.kt:8)
at io.mockk.impl.recording.CommonCallRecorder.done(CommonCallRecorder.kt:42)

Also tried just to gather information:

• running inside JVM test folder. It gets mocked without issues, but I can't run my test as JVM
• running androidTest inside Foo module. Got the same crash
• using mockkClass(Foo::class). Got some crash
• using annotation @MockK and MockKAnnotations.init(this). Got some crash.
• added Log.d before every { line and inside getData() method and it seems the actual real method from the class is getting called during the mock setup. That seems super weird to me.

Any idea what's going wrong here?

edit:

as requested, full code. I'm current working on an isolated project to try to isolate the error, so Foo is just:

class Foo {

    fun getData(): String {
        Log.d(TAG, "invoked foo.getData()")
        return "trolololo"
    }

}

and then I have FooTest in androidTest:

class FooTest {

    @Test
    fun mock_foo() {
        val foo = mockk<Foo>()
        every { foo.getData() } returns "zero"
        assertEquals("zero", foo.getData())
    }

}

It seems to be a Mockk opened issue: https://github.com/mockk/mockk/issues/182

2 possible quick fixes (pick one):

1. Run the Instrumented Tests in an emulator >= Android-P
2. Set Foo class as open (and the method(s) you want to mock too)