# 500 Server Error When Adding a Field to a Project

This article examines an error that might occur when you add a custom field to a project using YouTrack REST API. You might get this error when you reference a field prototype instead of a project custom field in a POST request.

## Error Message

```
{
    "error": "server_error",
    "error_description": "class jetbrains.charisma.customfields.rest.CustomFieldMegaProxy
    cannot be cast to class jetbrains.charisma.customfields.rest.ProjectCustomField
    (jetbrains.charisma.customfields.rest.CustomFieldMegaProxy and
    jetbrains.charisma.customfields.rest.ProjectCustomField are in unnamed module of
    loader org.eclipse.jetty.webapp.WebAppClassLoader @494dd782)"
}
```

## Request Details

Here are the details about a sample request that might result in this error.

| Attribute | Value |
| --- | --- |
| Request method | POST |
| Endpoint | `/api/admin/projects/projectID/customFields` |
| Request body |     ```JSON {     "name": "Last message related emails",     "id": "46-21",     "$type": "CustomField" } ```    |
| HTTP response code | 500 |

## Troubleshooting

Here are some possible causes and suggestions on how to overcome the error and add a field to the project.

### Cause

The ID of the custom field that you're using in the body of the POST request is incorrect. It references the prototype of a custom field instead of the project custom field.

In your request, you are adding a [prototype of a custom field](api-concept-custom-fields.html#custom-field) to a project. Projects, however, don't contain direct references to the base entity. Instead, they contain references to [instance](api-concept-custom-fields.html#project-custom-field) of a `CustomField` entity, which is `ProjectCustomField`.

### Solution

The first step to a valid POST request is to figure out which entity type this resource expects. In the case of the `/api/admin/projects/projectID/customFields` resource, the expected entity type is [ProjectCustomField](resource-api-admin-projects-projectID-customFields.html). You must pass an object of the corresponding type in the request body.

Each project custom field contains the settings of its custom field prototype (an object of the `CustomField` type). These settings are stored in the `field` attribute of the `ProjectCustomField` entity.

When you received the error, you tried passing an object of the `CustomField` type in the request body. Let's reuse this object in an attribute of a `ProjectCustomField` entity.

The final request body would look like this:

```JSON
{
    "field": {
        "name": "Last message related emails",
        "id": "46-21",
        "$type": "CustomField"
    },
    "$type": "ProjectCustomField"
}
```

Note that you still need to pass the correct `$type` of each object. The type of the main object is `ProjectCustomField`, and the type of the custom field prototype inside it is `CustomField`.