Why are you using string()? Is it a number field? Is it a select multiple?
I have seen it be picky on some field types. Also make sure to have spaces in there just in case.
try some of these options
${REST_TYPE} != ''
${REST_TYPE} != ""
string-length(${REST_TYPE}) != 0
string-length(${REST_TYPE}) > 0
If a select try
count-selected(${REST_TYPE}) != 0
count-selected(${REST_TYPE}) > 0
hope one of those does it.