Warning: Failed prop type: Invalid prop `defaultTitle` of type `array` supplied to `HelmetWrapper`, expected `string`.

[mariosffx]

Fixed by changing

<FormattedMessage id="title">
 {(txt) => (
  <Helmet defaultTitle={txt} titleTemplate={`%s | ${txt}`}>
       ....
  </Helmet>
</FormattedMessage>

to

<FormattedMessage id="title">
 {(propsTxt) => {
   const txt = propsTxt[0];
  <Helmet defaultTitle={txt} titleTemplate={`%s | ${txt}`}>
       ....
  </Helmet>
</FormattedMessage>

File: layout.js

layout.txt

[hugomn]

Hey @mariosffx! Sorry, just saw this now. Thanks for reporting. I'll add and push it! ;)