PHP Function inside of if statement appearing as having undefined error [duplicate]

This question already has an answer here:

• “Notice: Undefined variable”, “Notice: Undefined index”, and “Notice: Undefined offset” using PHP 27 answers

This only appears when I have WordPress in debug mode, but I'm still concerned about it. Basically, I'm using the plugin The Events Calendar with a bunch of custom fields. Want them to display in the template but only if the field has a value selected.

So I wrote this in my template:

<?php if( $additional_fields[ 'Crowds' ]) { echo '<i class="fas fa-users"></i> ' . $additional_fields[ 'Crowds' ] . ' Crowds' ; } ?> 

And it came back with the following:

Notice: Undefined index: Crowds in /home/hellod8/public_html/wp-content/themes/dadventure/tecshortcode/custom.php on line 30

So I'm trying to figure out how to correct this or it's just a FOL. I also did try to include

if( $additional_fields[ 'Crowds' ] == "0"

or

if( $additional_fields[ 'Crowds' ] == ""

but no luck on any of them.