...
After adding the missing plus sign, the value saves successfully.
Known Issues
One issue found in testing that will be resolved in the future, but is open at this time is that in some situations if you make your groovy into multiple lines with plus signs you may get the error:
[No signature of method: java.lang.String.positive() is applicable for argument types: () values: [] Possible solutions: notify(), tokenize(), size(), tokenize(), size()].
If you get this error, move your code onto a single line.