Getting "Warning: array_key_exists()" at the top of the page

I’m getting the following warning:

Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /wp-content/plugins/gravity-forms-salesforce/salesforce.php on line 438

if((!empty($data[$label]) && !empty($value) && $value !== '0') || empty($data[$label]) && array_key_exists($label, $defaults)) {
				   		$data[$label] = $value ;

I’ve seen some similar issues with other WordPress plugins and the popular solution appears to be to wrap the $label in single quotes like this: ‘$label’. Any suggestions?