JooQ - groupBy with function issue - ORA000979

gistfile1.java

        public static Field<String> toChar(Field<Timestamp> timestampField,
			String format) {

		return Factory.function("to_char", String.class, timestampField,
				Factory.val(format));
	}

	@Test
	public void testToChar() {
		
		Field<String> tochar = toChar(MZ_LOGIN_STATISTICS.TIME_IN, "MON");

		factory.select(Factory.count(), tochar).from(MZ_LOGIN_STATISTICS)
				.join(MZ_USER_DETAILS)
				.on(MZ_LOGIN_STATISTICS.USER_ID.equal(MZ_USER_DETAILS.USER_ID))
				.groupBy(tochar).fetch();
	}