Case sensitivity & array index

[Streusel]

In https://github.com/GeSHi/geshi-1.0/blob/master/src/geshi.php#L3382
We're provided with the following line:
$case_sensitive = $this->language_data['CASE_SENSITIVE'][$k];
The issue is that we cannot simply assume that the user 100% of the times has setup their files properly. This isn't a critical error but is one nonetheless.
My 'fix' for this instance, would be to check if the key exists

if(isset($this->language_data['CASE_SENSITIVE'][$k]))
   $case_sensitive = $this->language_data['CASE_SENSITIVE'][$k];
else
   $case_sensitive = false;