ExtractValue function doesn't work in stored procedure first few times

I have a stored procedure wich process an XML field to get some data. The first few times the procedure is executed it returns NULL or empty string, then it works fine. From HeidiSql, for example, it usually fails two times and then works right, and from ToadSql it fails about four times and then works everytime (the number of failures seems consistent, but could be luck).

The line that seems to be failing is a call to ExtractValue (here with some context):

DECLARE counter INT UNSIGNED DEFAULT 1;
WHILE (counter <= total_count) DO
SELECT ExtractValue(xml, '//service[$counter]/name') INTO service_name;
SET services = CONCAT(services, ',', service_name);
SET counter = counter + 1;
END WHILE;