Ensuring Unique Primary Key

For our database application we get our primary keys by calling a stored procedure that pass's in the table name and outputs the next primary key number assigned to that table.



ALTER procedure [dbo].[sp_getNextKey]

@TableName char(100),

@NextKey T_ID output

as

SET TRANSACTION ISOLATION LEVEL READ COMMITTED

begin transaction

select @NextKey = NextKeyValue from T_KeyGenerator where TableName like @TableName

update T_KeyGenerator set NextKeyValue = NextKeyValue + 1 where TableName like @TableName

commit transaction


I then take that primary key, and other data, and insert that into the desired table.

The Problem I am having is that my user's keep getting "cannot insert duplicate key in table".
So I assume that I do not have the transaction set right, or missing something.
I need it to lock the row in the t_keyGenerator table so that no other users can view that row until I update it with the new value and commit the transaction.

Any help would be greatly appreciated