Loading two fonts with one class gives an error

[jonathanstegall]

Using this code:

.pipe(
  postcss([
    foftLoadedClasses({
      groups: [
      {
        families: ["ff-meta-serif-web-pro", "ff-meta-web-pro"],
        classNames: ["fonts-loaded"]
      }
    ]
    }) // font loading
  ])
)

gives an error:

Message:
Cannot read property 'split' of undefined
Details:
fileName: style.css
domainEmitter: [object Object]
domainThrown: false

But when I use only one font, it works fine:

.pipe(
  postcss([
    foftLoadedClasses({
      groups: [
      {
        families: ["ff-meta-serif-web-pro"],
        classNames: ["fonts-loaded"]
      }
    ]
    }) // font loading
  ])
)

Am I misunderstanding the syntax for multiple families?