In the case of numerical attributes, TaurusValuelineEdit.getValue() returns None when the value does not validate according to the line edit's validator.
This is ok if the text is not a valid quantity representation, but it is not ok when the text is a correct quantity representation and the non-validation is due to it being out-of-range.
In this later case, the quantity should be returned.
I will propose a patch soon.
Note that fixing this may break some logic depending on the previous (incorrect) behaviour.
Proposed solution in https://github.com/cpascual/taurus/pull/33