Simple function returning a value is flummoxing me. worked once. (mysql)

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SQL

This MAY actually be my first question. So, here 'goes.

"I swear this worked initially and doesn't work now." (take that for all it's worth.)

I have a table, as follows:

CREATE TABLE `meta_data` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `data_key` varchar(64) NOT NULL,
  `data_value` varchar(64) NOT NULL,
  PRIMARY KEY (`id`)
) 

It has exactly one row, thus:

1,'current month','2014-05'

The idea is to create a function (a functional MySql object that returns a single value) that will retrieve the current month value. Easy peasy, right?

delimiter $$
create function f_current_run_date() 
returns varchar(16)

BEGIN

	declare current_run_date varchar(16);
	SELECT data_value into @current_run_date from meta_data where data_key = 'current month' limit 1;
	return current_run_date;

END$$

select 'current run date',f_current_run_date();$$

I swear to you that when I wrote this and executed this whole block, it worked fine. 10 minutes later, after an impromptu meeting, it returns null. When I extract the sql, (pulling out the 'into' clause) works exactly as I'd expect.

What gives? I'm beginning to feel like I'm insane here.

(This is MySql 5.1 btw.)