function inside a Jenkinsfile, Jenkins complains that `java.lang.NoSuchMethodError: No such DSL method 'anchor' found among steps`

gistfile1.txt

/* The expected JSON:
{
    "text": "Division by zero",
    "anchor": {
        "line": "59", 
        "path": "App/FifteenMinuteTest.m"
    }
}
*/

import groovy.json.JsonBuilder

private String buildJsonCommentFromCompileWarning(warning, baseCodePath) {
    def fileDetails = warning.file_path.find(/${baseCodePath}.*/)
    def splitFileDetails = fileDetails.split(":")
    def line_number = splitFileDetails[1]
    def file_path = splitFileDetails[0]
    def reason = warning.reason
    def builder = new JsonBuilder()
    def json = builder {
         text reason
         anchor {
            line line_number
            path file_path
        }
    }
    def payload = builder.toString()
    echo payload
    return payload
}

java.lang.NoSuchMethodError: No such DSL method 'anchor' found among steps [AddInteractivePromotion, ArtifactoryGradleBuild, ArtifactoryMavenBuild, ConanAddRemote, ConanAddUser, InitConanClient.....

I feel like lines 20-26 should give me the json object I want. However, after running my Jenkinsfile, I get the error at the bottom of the gist.

Isn't this the correct syntax for using JsonBuilder?