I've created a custom variable for my rule to parse the numerical value from the initiating issue key so that I can then later use in a mathematical operation to assign either tasks or sub-tasks based on the value.
My SmartValue is updating correctly however when I try to populate into the custom field, I get an error: "Operation value must be a string". I've placed debug into the code as well and it clearly shows that the end result is a string.

Code in Advance Automation section (for first issue create):
{
"fields": {
"Parent Link": {
"value": {{#debug}}"EI-{{#=}}{{NumericalKey}}+1{{/}}{{asJsonString}}"{{/}}
}
}
Any recommendations?