version of SQL Server instance MSSQLSERVER does not match the version expected by the SQL Server update

While trying to patch SQL Server 2008 R2 SP1 CU4 with CU6, I got the following prompt

It specifically says the below:
"The version of SQL Server instance MSSQLSERVER does not match the version expected by the SQL Server update. The installed SQL Server product version is 10.50.1600.1, and the expected SQL Server version is 10.51.2500.0"


This error says that I don't have SQL Server SP1 installed. This is rather interesting as I know for a fact I have SP1 installed. What to do?!!

Well I decided to reapply SP1 and see if there was a repair option. There wasnt a repair option, but on the config screen, I saw it had "Incompletely Installed" as the Upgrade Status. I ran it, and it quickly completed.

After the install completed, I retried to apply the latest CU patch, and voila!! It worked!!.