[Resolvido] Erro no Group By

olá pessoal,

estou com um problema no group by, pessoa a ajuda de voces,

SELECT nome_prod, data_inicio FROM tbformula WHERE (convert( nvarchar, hora , 111) BETWEEN ('2009/05/03 00:00:00') AND ('2009/05/05 23:59:59')) GROUP BY nome_prod
o erro:

Column 'tbformula.data_inicio' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.