Null values when applying correlation function over a Window?

Question

Having the following DataFrame I want to apply the corr function over the following DF;

    val sampleColumns = Seq("group", "id", "count1", "count2", "orderCount")

    val sampleSet = Seq(
      ("group1", "id1", 1, 1, 6),
      ("group1", "id2", 2, 2, 5),
      ("group1", "id3", 3, 3, 4),
      ("group2", "id4", 4, 4, 3),
      ("group2", "id5", 5, 5, 2),
      ("group2", "id6", 6, 6, 1)
    )

    val initialSet = sparkSession
      .createDataFrame(sampleSet)
      .toDF(sampleColumns: _*)

----- .show()

+------+---+------+------+----------+
| group| id|count1|count2|orderCount|
+------+---+------+------+----------+
|group1|id1|     1|     1|         6|
|group1|id2|     2|     2|         5|
|group1|id3|     3|     3|         4|
|group2|id4|     4|     4|         3|
|group2|id5|     5|     5|         2|
|group2|id6|     6|     6|         1|
+------+---+------+------+----------+

    val initialSetWindow = Window
      .partitionBy("group")
      .orderBy("orderCountSum")
      .rowsBetween(Window.unboundedPreceding, Window.currentRow)

    val groupedSet = initialSet
      .groupBy(
        "group"
      ).agg(
        sum("count1").as("count1Sum"),
        sum("count2").as("count2Sum"),
        sum("orderCount").as("orderCountSum")
    )
      .withColumn("cf", corr("count1Sum", "count2Sum").over(initialSetWindow))

----- .show()

+------+---------+---------+-------------+----+
| group|count1Sum|count2Sum|orderCountSum|  cf|
+------+---------+---------+-------------+----+
|group1|        6|        6|           15|null|
|group2|       15|       15|            6|null|
+------+---------+---------+-------------+----+

When trying to apply the corr function, some of the resulting values in cf are null for some reason:

The question is, how can I apply corr to each of the rows within their subgroup (Window)? Would like to obtain the corr value per Row and subgroup (group1 and group2).