Ambiguous self join error in non self join with window

Following code fails ambiguous self join analysis, even when it doesn't have self join 

val v1 = spark.range(3).toDF("m")
val v2 = spark.range(3).toDF("d")
val v3 = v1.join(v2, v1("m").===(v2("d")))
val v4 = v3("d");
val w1 = Window.partitionBy(v4)
val out = v3.select(v4.as("a"), sum(v4).over(w1).as("b"))

org.apache.spark.sql.AnalysisException: Column a#45L are ambiguous. It's probably because you joined several Datasets together, and some of these Datasets are the same. This column points to one of the Datasets but Spark is unable to figure out which one. Please alias the Datasets with different names via `Dataset.as` before joining them, and specify the column using qualified name, e.g. `df.as("a").join(df.as("b"), $"a.id" > $"b.id")`. You can also set spark.sql.analyzer.failAmbiguousSelfJoin to false to disable this check.;