Use of MERGE reserved keyword in different versions of SQL Server

I am getting runtime error while calling a stored procedure ( as mentioned below) in SQL Server 2012 but not in SQL Server 2014, it looks very fudamental but would like to know exact reason and if any work around.

Runs in SQL Server 2012

SP_AddState 29,merge

Gives error in SQL Server 2014 as

System.Data.SqlClient.SqlException (0x80131904): Incorrect syntax nearthe keyword 'merge'.

It works only when called as

SP_AddState 29,'merge'

So what is the reason of this execution behavior?