From a user's perspective, the value is 0.001 instead of 0.

steps to reproduce

1. load the attached zul
2. click the button

actual result

the message "error number is not allowed" shows

expected result

no error message shows.

workaround

Specify format with more precise pattern.

<doublebox id="db" constraint="no zero" format="#.###" />

root cause

getValue() calls checkUserError(), and it validates the value by coerceToString(value) according to default format #.#. So the value it gets is 0.0 which violates the constraint.

It's better to validate the real value without converting to a string first.