Dynamic Spark code Fails validate

When I create a pipeline in CDAP studio / data fusion and add a dynamic spark transformation with spark scala code inside. A simple 

```
def transform(df: DataFrame) : DataFrame = {
  df
}
```
Fails the validation.


The validation error is:

```
Errors
org.apache.spark.api.java.function.Function
```

What is the problem?

thanks!