Use of undefined constant &nbsp – assumed ‘&nbsp ‘

I have a line of code that looks like this:

<input type="<?= ( is_numeric( $antwort ) ) ? 'number' : 'text'; ?>" class="antwort" value="<?= $antwort ?>" <?= ( $i == 1 ) ? 'disabled' : ''; ?>>

The html output:

<input type="number<br />
<b>Warning</b>:  Use of undefined constant &nbsp; - assumed '&nbsp;' (this will throw an Error in a future version of PHP) in <b>/home/cardiodemowerbea/docroot/web/wp-content/plugins/cardio-score/admin/patients.php</b> on line <b>295</b><br />
" class="antwort" value="1">

Why is there a warning?
Where am I missing an &nbsp;?