`slug` is `undefined` when I track mark and children from internalLink

internalLink - mark 
_key: "7e6ceaaccca5"
_type: "internalLink"
reference: {…}
    _ref: "6230af03-9c14-4316-aa6f-ba7903d72c51"
    _type: "reference"

internalLink - children 
_key: "7e6ceaaccca5"
_type: "internalLink"
reference: {…}
    _ref: "6230af03-9c14-4316-aa6f-ba7903d72c51"
    _type: "reference"

_rawText(resolveReferences: {maxDepth: 10})

  marks: {
    strong: (props) => <strong>{props.children}</strong>,
    em: (props) => <em>{props.children}</em>,
    internalLink: ({ mark: { reference }, children }) => {
      const { slug = {} } = reference;
      const href = `/${slug.current}`;
      return <Link to={href}>{children}</Link>;
    },
  },

internalLink: ({mark, children}) => {

internalLink: ({ mark: { reference }, children })