error messages for formula field in items/items prototype not consistent

different messages appear if formula field is ticked but value box is left empty (default value is deleted, as default value is "1")
when clicks save on:

-items -page will refresh and error message will appear "Warning. Field "Custom multiplier" is mandatory."
at the same time value "1" (default) will appear in formula field

-item prototype -page will refresh and error message will appear "Warning. Field "Custom multiplier" is not decimal number."
and no default value ("1") will appear in formula field