Dynamic Query in Stored procedure of mysql call giving error as Truncated Incorrect DOUBLE value

this is Procedure:

delimiter $$

CREATE DEFINER=`root`@`localhost` PROCEDURE `SampleSP`(IN uname VARCHAR(50),OUT uid VARCHAR(50))
BEGIN

DECLARE sqlcmd VARCHAR(500);
set sqlcmd = 'SELECT userid into '''+uid+''' FROM usertable WHERE username = '''+uname+'''';

EXECUTE sqlcmd;

END$$

I am calling it as..

use `test`;

call SampleSP('Adam',@uid);

its giving error as..

Error Code 1292. Truncated Incorrect DOUBLE value: '''

Im not sure whether I am syntactically correct.