NumberField and illegal characters

NumberField (at least in Firefox) allows input that cannot be parsed to a number. But on the server-side, the value only changes to null. So my application is basically clueless about the invalid input. It might only complain that the field is required, which is confusing when there is clearly something in the field on the client-side. On a Vanilla <input type="number"> Firefox would at least create a red outline and add a tooltip to please enter a number. In Vaadin the tooltip is there (if one happens to hover over the field by any chance), but the outline (for obvious reasons) is not. So how can I indicate that somethings wrong or provide a helpful error message?
BigDecimalField is more restrictive in its allowed input, but will behave in the same way if you enter multiple decimal points.