Thanks Amandeep. This seems like a valid bug to me as quoted columns are not handled property for na.fill(). I think the better place to fix is in DataFrameNaFunctions.scala <https://github.com/apache/spark/blob/18b30107adb37d3c7a767a20cc02813f0fdb86da/sql/core/src/main/scala/org/apache/spark/sql/DataFrameNaFunctions.scala#L422> where "f.name" should be quoted.
Could you create a JIRA <https://issues.apache.org/jira/projects/SPARK/issues>? Thanks, Terry
