diff --git a/docs/sql-programming-guide.md b/docs/sql-programming-guide.md
index ccd26904329d3da0fffcee8ba7b050a11cf7912c..6e02d6564b0021dbe5bdc23efcad2b63b0ed92d2 100644
--- a/docs/sql-programming-guide.md
+++ b/docs/sql-programming-guide.md
@@ -982,7 +982,8 @@ when a table is dropped.
 
 [Parquet](http://parquet.io) is a columnar format that is supported by many other data processing systems.
 Spark SQL provides support for both reading and writing Parquet files that automatically preserves the schema
-of the original data.
+of the original data. When writing Parquet files, all columns are automatically converted to be nullable for 
+compatibility reasons.
 
 ### Loading Data Programmatically