Cannot set one Custom Field to another Custom Field, what am I doing wrong?

This seems like a simple thing, but I cannot assign my custom field Severity to Priority field. Here's my code: 

const entities = require('@jetbrains/youtrack-scripting-api/entities');
const workflow = require('@jetbrains/youtrack-scripting-api/workflow');

exports.rule = entities.Issue.onChange({
title: 'If Bug type ticket, map Severity changes to Priority field',
guard: (ctx) => {
return ctx.issue.fields.isChanged(ctx.Severity) && ctx.issue.fields.is(ctx.Type, ctx.Type.Bug);
},
action: (ctx) => {
const issue = ctx.issue;
issue.fields.Priority = issue.fields.Severity;
},
requirements: {
Type: {
type: entities.EnumField.fieldType,
Bug: {
name: 'Bug'
}
},
Severity: {
type: entities.EnumField.fieldType,
ShowStopper: {
name: 'Show-stopper'
},
Critical: {
name: 'Critical'
},
Major: {
name: 'Major'
},
Normal: {
name: 'Normal'
},
Minor: {
name: 'Minor'
}
},
Priority: {
type: entities.EnumField.fieldType,
ShowStopper: {
name: 'Show-stopper'
},
Critical: {
name: 'Critical'
},
Major: {
name: 'Major'
},
Normal: {
name: 'Normal'
},
Minor: {
name: 'Minor'
}
},
}
});