Can't get my function to work on MySql

Can you try this?

CREATE FUNCTION GetPartnerFieldValue(partnerId BIGINT, fieldCode VARCHAR(255))
RETURNS VARCHAR(255)
DETERMINISTIC
BEGIN
	DECLARE sReturnString VARCHAR(255);

		SELECT
			f.value
		INTO
			sReturnString
		FROM 
			PARTNERFIELD f,
			PARTNERFIELDTYPE ft
		WHERE
			f.partner_id = @partnerId
			AND f.partnerFieldType_id = ft.id
			AND ft.code = @fieldCode;

	RETURN sReturnString;
END;

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Same problem:

Error at line 5, error in sql syntax.
Undeclared variable sReturnString.
Syntax error on the return line;

How about this?

CREATE FUNCTION GetPartnerFieldValue(partnerId BIGINT, fieldCode VARCHAR(255))
RETURNS VARCHAR(255)
DETERMINISTIC
BEGIN
	DECLARE sReturnString VARCHAR(255);

	SELECT * INTO
			sReturnString FROM
		 (
		SELECT
			f.value
		FROM 
			PARTNERFIELD f,
			PARTNERFIELDTYPE ft
		WHERE
			f.partner_id = @partnerId
			AND f.partnerFieldType_id = ft.id
			AND ft.code = @fieldCode
	) result;

	RETURN sReturnString;
END;

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Still the same.

It looks to me that the DECLARE function isn't working.
I get the first error when declaring the sReturnString variable.
And I think this causes the errors for rest of the script.