Scala Spark unit test Dataset.map function

Question

I am having trouble writing unit test for the Dataset.map function for the below snippet, I am trying to write unit test using Flatspec and Matchers library

// Function within a Class
def dataValidation(ds: Dataset[someCaseClass]): Long = {

    val validDs: Dataset[Booelan] = ds.map(entry => validate(entry))

    validDs.filter(entry = !entry).count

}

def validate(data: someCaseClass): Boolean{
  if(valid(data))
     true
  else
     false
}

In the above code snippet I am facing hardtime in writing mock data for

val validDs: Dataset[Booelan] = ds.map(entry => validate(entry))

I get below error if I try to mock the map function as below

unit test code
when(
      mock[Dataset[someCaseClass]].map(entry => validate(entry))
    ).thenReturn(mock[Dataset[Boolean]])

error that I get

SmartNull returned by this unstubbed method call on a mock:
dataset.map(
    <function1>,
    class[value[0]: boolean]
);