GROUPBY IFNULL - column not accessible in HAVING

When IFNULL is used in GROUPBY, Unknown column error is thrown when trying to use it in HAVING....

To reproduce:
CREATE TABLE aaaa (a INT, b INT, c int);

SELECT 0 FROM aaaa GROUP BY IFNULL(a,b) HAVING IFNULL( aaaa.a, aaaa.b)<>SUM( IFNULL( aaaa.c, 0));

Result: Error - Unknown column 'aaaa.a' in 'having clause'

Strange is that when SELECT * is used works ok...