Cannot add a SimpleContent column to a table containing element columns or nested relation

This is a classic case of an error message that is not exactly a misleading one, but one that must be interpreted properly.

This is the story. In a WCF application, I added a dataset based on T-SQL query. Everything worked as expected. In my tests, I was able to fill a dataset, and send it back to WCF client. The dataset was a public member of a class instance.

My next step was moving the actual query to a stored procedure. The purpose was to keep consistent design across the WCF application. All dataset in the app were designed to call respective stored procedures, rather than execute direct SQL SELECT statements.

This is were my WCF client started to throw an exception: Cannot add a SimpleContent column to a table containing element columns or nested relation.

I admit giving only a little effort understanding the error message. And yes, I googled the text of the error message, a lazy approach. Then I tried to change the stored procedure first to an inline function, and then to a multi-statement function. As you understand, the outcome was exactly the same.

The actual System.Data.SqlClient.SqlException error, and I had to caught it on WCF server side, was caused by missing SELECT permission on SQL Server function I created. So there was no valid dataset returned with WCF server's responses, which subsequently was causing deserialization fail on WCF client side. Simple!