Passing dynamic arguments to gman_do mysql udf function

Question

I am trying to use gearman's mysql udf functions (version 0.6) .

When i call it like this :

select gman_do_background("test", "_", "_"); 

or even

select gman_do_background(CONCAT("te","st"), "_", "_"); 

It works, and my worker "test" procedure is triggered.

But if i try to do :

CREATE DEFINER  PROCEDURE `CallGearmanBackground`(IN `jobtocall` VARCHAR(70))
     BEGIN select gman_do_background(jobtocall, "_", "_" );
END 

and then

call CallGearmanBackground("test"); 

I get this error :

Can't initialize function 'gman_do_background'; First argument must be a string.

I tried several type for the procedure argument (VARBINARY, TEXT, BLOB ) ... all trigger the same error.

Any way around this ?