Natalie Dessay/Concerto Koln, Barbican Hall, London

There's a touch of Piaf about Natalie Dessay. A more glamorous variety of songbird than the "little sparrow", it has to be said, but she arrived on stage here looking decidedly fragile and apprehensive. She clutched the ebullient conductor Evelino Pido's hand; she coughed nervously; she didn't smile; she wasn't happy. But then nor was Mary, Queen of Scots contemplating her future from the grounds of Fotheringhay Castle in act two of Donizetti's Maria Stuarda.

That was Dessay's first "number" here and she embraced its line with sensitivity, shape and purpose. Addressing a cloud, Mary's line "In kindness descend and take me on your wings" brought a perceptively airy turn to the coloratura. But all was not well in the "remote" upper reaches of the voice when Dessay dared to take the sound away, to attenuate the line pianissimo for expressive and dramatic effect. Each time she attempted it, a "catch" in the sound broke the thread – the voice simply didn't "speak". Perhaps she was shaking off a cold, perhaps she was tired? But given her past vocal problems the question must at least be asked: has this featherweight soprano voice been beefed up for stardom? Is too much being asked of it?

She would never (or should never) sing Gilda in Verdi's Rigoletto in the theatre, for instance, but she can sing the aria "Caro nome" and did so here with cosy, warm-all-over rapture. There was bliss in the little trills, rash excitement in the sudden shrieks of delight above the stave. But the high D was pushed, not floated (another tell-tale sign), and before we could give that another thought she had hypnotised us by leaving the stage on an eternal final trill.

The point is that Dessay is a wonderful actress and can disguise her vocal shortcomings as few can. She almost convinced us that Violetta's act one rollercoaster from La Traviata was not a stretch for her. Emotionally it isn't, of course, and we could read every last vestige of conflict, indecision and deception in her face and body language. And she certainly gave us the wanton abandon vocally. But she must nurse her talent and think seriously about what and where she sings. No Violetta in the theatre, please.