In the description of the 'stoichiometry' attribute, the spec reads:
"The stoichiometry attribute is of type double and should contain values greater than zero (0)"
However, while there is a validation error for the first bit (21114), there is no validation warning (that I could find) that covers the second bit ('it should be greater than zero'). We should add one!
EDIT: The consensus was to not add a validation rule, and instead to remove the "should contain values of greater than zero" line.