Format command has incorrect syntax causing "string required" error

Terraform
1

What is the correct syntax for the following format command as referenced in locals.tf?

 Error: Error in function call
   on modules/X/locals.tf line 7, in locals:
    7:   node_labels = format("%s=%s", var.node_labels_prefix, [element(split(":", var.availability_domain_id),1)])
     ├────────────────
     │ var.availability_domain_id is "XXXX:YYYYYYYY"
     │ var.node_labels_prefix is "PREFIX"

 Call to function "format" failed: unsupported value for "%s" at 3: string required.

I expect the following return.

PREFIX=YYYYYYYY
2

Resolved with the following syntax

 node_labels = format("%#v=%#v", var.node_labels_prefix, element(split(":", var.availability_domain_id),1))

which produces the expected return.

PREFIX=YYYYYYYY